Need Help With a Tuner

[mwezell]

Well that was clear thank you. I have been moving too far it appears and for years. I believe a 30 ppc and a 6 ppc has a different movement requirement. It could be the hole size in the barrel or a combination of things. But from your work I will create a test plan, have it shot in a tunnel, and in open air one thousandth at a time. This I have not done on a 6mm that finely, but have done it on a 30.

I will set up a gun the very next time to the range with good conditions. First shoot a tune test one thousandth at a time.
then use that tune to establish a cycle I call span (node to node)
then with the gun in tune trace the tuner change required to stay in tune each hour and log Density altitude, Temp and Humidity as I go. That should give us an Idea of click per DA unit.
Then repeat that for the two main powders several times. Then buy more barrels because that will burn up some on the way. Grrrrrrr.
I will let you know what I find.

thank you for all your help.

Stewart

Great! Keep us posted.

 

[mwezell]

Well that was clear thank you. I have been moving too far it appears and for years. I believe a 30 ppc and a 6 ppc has a different movement requirement. It could be the hole size in the barrel or a combination of things. But from your work I will create a test plan, have it shot in a tunnel, and in open air one thousandth at a time. This I have not done on a 6mm that finely, but have done it on a 30.

I will set up a gun the very next time to the range with good conditions. First shoot a tune test one thousandth at a time.
then use that tune to establish a cycle I call span (node to node)
then with the gun in tune trace the tuner change required to stay in tune each hour and log Density altitude, Temp and Humidity as I go. That should give us an Idea of click per DA unit.
Then repeat that for the two main powders several times. Then buy more barrels because that will burn up some on the way. Grrrrrrr.
I will let you know what I find.

thank you for all your help.

Stewart

Ralph, I agree that tuner movement will change a small amount due to barrel stiffness between a 22,6mm and a 30...assuming that the length and contour are identical. IME, it doesn't change much, though. I'll only speak in specifics in regard to my tuners..but what I have seen is that the 4 marks will hold pretty true, given the same length and contour, regardless of caliber. Instead of 4 marks, you may see the magic number being 3 or 5, depending on barrel stiffness mostly. Nevertheless, the 4 marks that I mentioned is not meant to be precise, nor exactly true on every gun..much less every tuner.

I have mentioned several times in this thread that I'd like to see us get to the point where we can refer to barrels with a relative stiffness number. Dan Lilja just happens to have a stiffness calculator on his site. This alone will not address ALL of the variables in tuner discussions, but will be a step in the right direction to talking apples to apples regarding tuner adjustments, IMHO.

I have played with what Boyd mentioned earlier..a hinge point machined into the barrel that makes it less stiff. I don't have numbers but can say that it did make a noticeable difference in how the gun tuned. The idea was and is to be able to get the same effect as a heavier tuner but with less tuner weight. Obviously, this opens a Pandora's box with all of the possibilities. Perhaps there will be an ideal barrel contour and length for a given tuner design within a given weight class.

 

[CubCouper]

Mike, your use and methods very much mirror what I've discovered and practiced since starting with Gene's tuners in 2008. I'd like to re-enforce your message that computer simulations are NOT results. I've had dozens of discussions with arm-chair accuracy experts who can quote every frame of Varmint Al's FEA analysis but don't get that a computer model is just that -- a model -- complete with assumptions and pre-supposed results.

Jackie, I couldn't agree more that the thing that prevents most shooters from finishing higher in the ranking is that they are not keeping up with the tune of their rifles... however they choose to tune them.

I started shooting in 2006. By the end of 2007 I had shot in 38 registered matches and had struggled most of that time with middle-of-the-pack finishes, but toward the end I had managed to win a couple yardages and my first two-gun. Over the winter of 2007-2008 I immersed myself in trying to figure out why certain people were consistent winners, while others like myself never quite had what it took to finish first.

I'm a data-holic (software developer by trade) and I had saved every match report and every target for those matches I had shot. I studied those reports. Not just my own scores, but the scores of every person who shot. It seemed the lots of folks could have won any yardage, but they didn't. Then it struck me that in most matches with reasonable conditions, the shooters in the top third or even top half of the field had exactly one group that likely determined where they finished in any given yardage. One group. If it was a .106, it meant they finished several places higher than some fellow competitors, and if it was a .278 or a .305 they had a nice finish, but likely didn't win. And if it was .356 or a .389 they were a lot closer to the middle of the pack. You usually don't see two bad groups until you get to the bottom half of the results page. The consistent winners seemed to have a 'bad' group too, but it was a .256, not a .356.

Armed with a few hints I'd overheard from top shooters like Mike Ratigan and Ed Adams and Charles Huckeba (I was still a new shooter in those days and didn't know those guys then), I started thinking about those patterns. One hint I'd heard was to change the load BEFORE the group blew up. In essence, recognize that the conditions are always changing and I needed to learn how to get ahead of the curve instead of waiting for a .342 to ruin my low 2 agg.

That year I was blessed with good shooting barrels and a passion for learning how to stay ahead of the tune curve. This was before I had ever seen a tuner, so my method was having a least a couple loads with variations of powder and/or seating depth to test on every target in order to not shoot that bad group with an out-of-tune load. I took copious notes and by the middle of that 2008 season I had worked out a complicated formula for adjusting powder measure and seating depth by watching the temperature. I was predicting the small changes necessary to keep the gun in tune for every target and the results were very positive: near 100% top 5 finishes, dozens of yardage wins, and at least 6 two-guns.

In September that year, I spent a day in Gene's tunnel to test my 'formulas' and showed Gene that I could reliably predict my group shapes based on my measure and depth tweaks. Gene just nodded courteously, and demonstrated that he could do the exact same thing with his new tuners. I shot his rifle and recognized that his tuner adjustments were definitely less haphazard than my screwy click&depth charts. So I had him fit one of my barrels with this new gizmo. That barrel had six top 10 finishes in the Nats a month later and top 20 finishes in 2, 3, and 4 gun (and yes I was shooting my one bag gun with my good barrel even in the unlimited marathons). Every shot fired with the same load. Making tuner adjustments instead of using my trusty charts.

Point here is that we shoot in very dynamic conditions: the days warm up or cool down, humidity changes, equipment and components dry out, or get moist, change volume, cook or freeze. Believing that there is a single, perfect load or tuner position that is good enough is the holy grail of shooters destined to finish in the middle of the pack. The winners know it is a fools errand, and that a truly peak tune is a moving target. They have some method for recognizing it and for staying on it every single target -- be it powder changes, neck tension, tuner settings, seating depth, or magic incantations. When I hear a guy yack about how perfect his tune is and he is in first place after three or even four targets, I would take odds almost every day on his load blowing up on the next target or two. The winner is likely in 2nd place and working hard to figure out what to change even though his last group was a .176.

Rod

 

[Steelhead1]

This is a fantastic thread, and I have learned a lot about tuners and what makes them tick. Several months ago I purchased a Harrells tuner for a smallbore rifle, and was completely amazed at what incredible results you can get with different ammo. Now I just bought one for my centerfire rifle, and will hopefully get it installed, and tested before leaving for Phoenix in 2 weeks. Thank you to everyone that has contributed to this thread it is a true indicator of how willing benchrest shooters with all levels of experience, are willing to share this valuable info.

Gary

 

[Boyd Allen]

I think that it should be mentioned that virtually nothing about Varmint Al's work addresses the issue of tuner adjustments made to stay in tune as conditions change. What he did initially was to create a model of a specific rifle tuner combination, make some projections based on that model, and then test the actual rifle to confirm that the model gave similar results. Once he had confirmed that his method of modeling gave results that were consistent with actual test results he continued on, using that method to explore other schemes to achieve what has been referred to as positive compensation. What he did was come up with a plausible explanation for how tuners work, and how positive compensation may be achieved by other means.

Another thing worth mentioning is that the most successful user of tuners in short range group competition that I know of, Gene Buckys, has told me, and published that he does not change his tuner setting after the initial adjustment. He finds that the advantage that a tuner gives is in broadening nodes so that it is easier to stay in tune using conventional means. I think that his results speak for themselves. This is not to imply that other methods do not work, but rather to point out that this particular cat has been successfully skinned using more than one method.

Yes, this has been a very good thread.

 

[CubCouper]

Varmint Al's models address the cantilevered-beam approach to describing what happens with a barrel when a gun is fired. My opinion is that this is more useful from a gun-handling and bench-bag-setup perspective than it is as a discussion of tuner dynamics. This is especially true when you try to scale his assumptions down to the micrometer-like adjustments that have been proven effective.

A lot of folks ask 'how does a tuner work?' and seem somewhat satisfied to start with the easily grasped cantilever-beam explanation. I think the big problem is that is it is the wrong question, and therefore asserts the wrong set of assumptions. If you ask 'how does rifle tuning work?' and forget the tuner for a moment, you realize that minuscule changes in either powder or seating depth or neck tension or tuner position can all have essentially the same effect... multiple solutions to the same problem, and this applies to nearly every rifle from a full-on railgun to a 6 pound mountain-lite hunting rifle. This says to me that mathematical/engineering calculations that pre-suppose vibration patterns, material stiffness, and weights are starting with some assumptions that are biased.

A good description 'how does rifle tuning work?' should offer a unified model that works regardless of the tuning method -- straight powder/depth changes and/or tune adjustments that incorporate mechanical adjustments to the gun like stationary or movable tuners. The tricky part is the scale of the changes: moving a tuner .002" can be shown to have the same effect as changing the seating depth by a similarly small amount. No cantilevered beam changes in the seating depth method, so barrel whip due to changing weight on the end of a lever is a no-go model for me.

I've also thought about why rifle tunes 'seem' to correlate to atmospheric conditions. I certainly track temp/humidity to analyze my tuner changes. Why do we even need to adjust for a 10 degree temperature change? A cartridge inside a warmed-to-hot barrel ought to be relatively independent of a mere 10 degree change past the end of the muzzle, but a 1/10th of a grain of powder (.3% difference at 28.6 grains) can change your group size from .360 to .120 (300%) -- that is a 1000x multiplier. Part of the answer is seeing a barrel as a hydraulic cylinder that is open on one end. The bullet has to push against the standard atmospheric conditions to clear the air ahead of it in the barrel.

I *currently* like the explanation given by Chris Long in his Acoustic Shock Wave work [found here]. In short, the ignition event shock wave causes a 'choke' to run up and down the barrel several times as the bullet is building up speed. The bottom line is that the most critical portion of a bullet's journey is the first quarter inch as it starts down the barrel. If you follow his shock wave theory, you can see that adjustments in seating depth, powder charge, neck tension, tuner position, and yes, even atmospheric conditions would all have some effect on the bullet at this critical point in the journey. In short, my interpretation is that we should tune for bullet-start, not bullet-exit. Within any single tuning node, we can make several kinds of changes that speed up or slow down the beginning of the bullet movement.

If you made it this far, here is 'model' that demonstrates this. I was arm-chair testing some loads using the QuickLoad software and I ran across a seldom-adjusted parameter for 'Shot Start (Initiation) Pressure'. The normal value for this is around 3600psi and represents the amount of pressure that builds in the case before the bullet starts to move. The documentation suggests that values in the range of 3600 to 6500 cover a variety of normal rifle bullet types... BUT... if the bullet is touching the rifling, you should add 7200psi. Lights should be going on here. We have a pressure value that we routinely adjust from 3,600psi to an upper limit of around 14,000psi (a 300% range) before the bullet starts to move! That is our tuning window. Using Quickload, the estimated peak pressures from a 6PPC shooting a 65gr bullet from a 21 inch barrel using 28.4 grains of N133 work out like this:

Initiation Pressure (psi)	Peak Pressure (psi)
3626	52367
5600	55316
7600	58111
9600	60769
11600	63318
13600	65778

That is a 20% variation in peak pressure that could be caused by seating depth or neck tension or shock wave timing or atmospheric resistance -- remember, I held the powder charge constant. Here is the peak-pressure vs. barrel-time charts for exactly the same load and tweaking only the factor for Initiation Pressure:

3600 PSI (i.e. just off the lands)

13600 PSI (i.e. hard jam)

My assertion: tuner discussions that center around the barrel flapping in the breeze are looking at the wrong end of the gun.

Rod

 

[Boyd Allen]

Well written, and it makes me want to ask a question. Do the computer generated differences in pressure (for the example that you gave) show up as proportionate differences in MV...in real world testing. As far as that goes, using that program, what calculated velocities were associated with those starting and peak pressures? With those numbers in hand, one could preload a test and fire it over a chronograph. The results might generate some illuminating data. Some time back, when I was at a match, I noticed one of the most experienced shooters in the sport do some tuning with seating depth, in contrast to the usual fiddling with a powder measure, or in his case Chargemaster. That made me wonder how small seating depth changes would compare to small charge weight (or volume) changes as far as tune is concerned. I would assume (guess) that such depth changes would have more dramatic per unit when made within the span from jam to touch, than if both settings involved jumping the bullet. Some time back, one of the most prominent crazy experimenters in the spot told me that seating depth has tuning nodes, both in and out of the rifling, just like powder charge.

 

[mks]

This says to me that mathematical/engineering calculations that pre-suppose vibration patterns, material stiffness, and weights are starting with some assumptions that are biased.

Rod,
I certainly agree that the computer models that have been applied so far do not include all of the factors that can affect accuracy, but I don't think it is fair to call them biased. That would imply that the model was purposely built to get a predetermined answer. I don't think that is the case. Rather, a computer model to include all the factors would take so much time to run that we would all be dead before the results got spit out. One makes choices to simplify the model that are quite necessary to get results in a reasonable time. Al's model was designed to explain compensation, and it does a nice job of that. Sorry, your choice of words raised my "anti-science" hackles a bit.

My assertion: tuner discussions that center around the barrel flapping in the breeze are looking at the wrong end of the gun. Rod

Have you calculated the change in bore diameter that an acoustic wave would cause, or the weight of air in front of the bullet? If not, please do, and see if your opinion changes. Even if these effects were large enough to have a significant effect, how do they translate into a different impact on the target? To explain tuning, we need a mechanism that affects both vertical and horizontal dispersion. Acoustic waves are axial waves, so they might affect the velocity of the bullet when it exits, but not its launch angle. This could affect vertical POI, but not horizontal. Thus, there is no way to explain why tuners adjust horizontal dispersion with the acoustic wave theory, unless of course, I am missing something.

Best,
Keith

 

[CubCouper]

The discovery of peak pressure sensitivity to the bullet start pressure got me to thinking along the same line. I believe there would be a disjointed curve that is fairly flat before touch, but has a sharp rise just where the bullet is touching the lands, then a fairly steep approach to the hard jam max initiation pressure. That would certainly explain why you either want to be in the rifling or out, but not on the hairy edge where slight differences in the could make one bullet touch and the next miss.

Another tuning technique that correlates to this is one used by several 'top-20' shooters. Shooting mostly in the mountain west, I rarely visit that rare 30+ grains of N133 area -- my cases would fry in just a few targets. In the lower nodes, I tend to decrease powder or seat the bullet slightly deeper as the temperature goes up (this from my tuning formulas prior to using a tuner). I was visiting with an eastern hall-of-famer who told me I was doing it backwards -- he let the bullet out as the temperature warmed up. After a bit more discussion, we figured out that we were starting from opposite ends of the tuning spectrum. With my light loads and western elevations, I tuned from the hard jam in. With his packed case of powder and thick eastern air, he started off the lands and gradually eased the bullet into the lands to manage his tune. He was just working on the back side of the curve. Again two different approaches to the same result. Side note: his use of a .269 neck (vs my .262 neck) gave him a fairly deep window of neck tension that probably also has a substantial impact on starting pressure.

Concerning the computer generated pressures, here are the predicted muzzle velocities: for the same range of Initiation Pressures, again all modeled with 28.4 grains of N133, 21" barrel, 65gr bullet... I'm only changing the initiation pressure:

Initiation Pressure	Peak Pressure	MV (fps)
3620	52384	3148
5600	55343	3171
7600	58138	3191
9600	60795	3207
11600	63345	3222
13600	65805	3234

So... that 20% rise in pressure also predicts an 86fps (2.6%) increase in MV. While measuring initiation pressure directly would be difficult, this would be easy to validate by chronographing a consistent powder charge over a series of seating-depth only adjustments (probably in the .001-.002 step size). Plotting MV from a few thou before touch through to a deep hard jam should give an actual picture of the curve I described above.

Interpolating from Long's Shock Wave Theory, I'd suggest that powder charge and seating depth are not the only factors that can impact this initiation pressure. Relating this to tuners, various steel models suggest the choke pulse of the shock wave could open (or close) the rifling diameter as much as .0005". This pulse makes 5-8 trips to the end of the barrel and back while the bullet is still in the gun (speed of sound in steel is roughly 19,000 fps). Changing the tuner by even a few thou changes the timing of the reflection and thus the timing of when the pulses are reflected back onto the chamber just as the bullet has started moving. A half-thou larger (or smaller) bore would certainly affect the required starting pressure for a bullet. Similarly, the bullet is pushing with nearly 2800 pounds of force just to move the air in the barrel ahead of it. Slight changes in atmospheric air density could vary the start condition as well.

For what it is worth, I use the Quickload software as my go-to tool for winnowing the field of possibilities when I do load development for a new gun. MV has typically been within 20-30 fps of the predicted values (as measured over an Oehler 35P) for a variety of hunting and varmint cartridges. It helps me identify loads with the trifecta of: 95+% fill ratio, 95%+ burn at 2/3-3/4 of the barrel length, and a peak pressure about 5-8% below max. Using this criteria, I have found numerous sub-MOA loads within a couple tries for calibers ranging from a .222 through a .375H&H.

Rod

 

[mwezell]

The discovery of peak pressure sensitivity to the bullet start pressure got me to thinking along the same line. I believe there would be a disjointed curve that is fairly flat before touch, but has a sharp rise just where the bullet is touching the lands, then a fairly steep approach to the hard jam max initiation pressure. That would certainly explain why you either want to be in the rifling or out, but not on the hairy edge where slight differences in the could make one bullet touch and the next miss.

Another tuning technique that correlates to this is one used by several 'top-20' shooters. Shooting mostly in the mountain west, I rarely visit that rare 30+ grains of N133 area -- my cases would fry in just a few targets. In the lower nodes, I tend to decrease powder or seat the bullet slightly deeper as the temperature goes up (this from my tuning formulas prior to using a tuner). I was visiting with an eastern hall-of-famer who told me I was doing it backwards -- he let the bullet out as the temperature warmed up. After a bit more discussion, we figured out that we were starting from opposite ends of the tuning spectrum. With my light loads and western elevations, I tuned from the hard jam in. With his packed case of powder and thick eastern air, he started off the lands and gradually eased the bullet into the lands to manage his tune. He was just working on the back side of the curve. Again two different approaches to the same result. Side note: his use of a .269 neck (vs my .262 neck) gave him a fairly deep window of neck tension that probably also has a substantial impact on starting pressure.

Concerning the computer generated pressures, here are the predicted muzzle velocities: for the same range of Initiation Pressures, again all modeled with 28.4 grains of N133, 21" barrel, 65gr bullet... I'm only changing the initiation pressure:

Initiation Pressure	Peak Pressure	MV (fps)
3620	52384	3148
5600	55343	3171
7600	58138	3191
9600	60795	3207
11600	63345	3222
13600	65805	3234

So... that 20% rise in pressure also predicts an 86fps (2.6%) increase in MV. While measuring initiation pressure directly would be difficult, this would be easy to validate by chronographing a consistent powder charge over a series of seating-depth only adjustments (probably in the .001-.002 step size). Plotting MV from a few thou before touch through to a deep hard jam should give an actual picture of the curve I described above.

Interpolating from Long's Shock Wave Theory, I'd suggest that powder charge and seating depth are not the only factors that can impact this initiation pressure. Relating this to tuners, various steel models suggest the choke pulse of the shock wave could open (or close) the rifling diameter as much as .0005". This pulse makes 5-8 trips to the end of the barrel and back while the bullet is still in the gun (speed of sound in steel is roughly 19,000 fps). Changing the tuner by even a few thou changes the timing of the reflection and thus the timing of when the pulses are reflected back onto the chamber just as the bullet has started moving. A half-thou larger (or smaller) bore would certainly affect the required starting pressure for a bullet. Similarly, the bullet is pushing with nearly 2800 pounds of force just to move the air in the barrel ahead of it. Slight changes in atmospheric air density could vary the start condition as well.

For what it is worth, I use the Quickload software as my go-to tool for winnowing the field of possibilities when I do load development for a new gun. MV has typically been within 20-30 fps of the predicted values (as measured over an Oehler 35P) for a variety of hunting and varmint cartridges. It helps me identify loads with the trifecta of: 95+% fill ratio, 95%+ burn at 2/3-3/4 of the barrel length, and a peak pressure about 5-8% below max. Using this criteria, I have found numerous sub-MOA loads within a couple tries for calibers ranging from a .222 through a .375H&H.

Rod

That settles it. I never want to hear again, how tuners complicate anything. Lol!

 

[CubCouper]

Keith -- I was busy writin' when you posted, so here is what I got. First, I don't mean to sound 'anti-science', in fact I'm quite the opposite -- a purist data analyst and programmer by profession. And I have verified the 2800# resistance of the air number and .0005 bore diameter change to the degree that I can (I'm a computer geek, not an engineer -- hence I refer to Chris Long's paper). I've read through Al's work several times over the last 8 years and whenever I thought I had a thorough refute, I studied a little more and usually found his statements reasonable in the context they were presented. But I've also had a number of folks cite his cantilevered-barrel model as the gospel of tuner dynamics. My (mostly) empirical analysis leads me to believe that it is only a small part of the unified understanding of tuning a rifle. And this is just my own muddled analysis, for others mileage may vary.

With the twisting torque reaction from the bullet spin, potential banana-shaped bores buried in our barrels, and stacking effects of the various vibration modes modeled in Al's work (I'm citing him now), measuring 'in-tune' as 'lack-of-vertical' leaves my picture of the results a bit short. Several years ago I was boasting to none other than Tony Boyer that I could "dial a zero" with my tuner and he ought to try it. Needless to say, Tony didn't need my advice . He did give me my first clue that perfectly flat was not necessarily perfectly in tune. Jack Neary filled in some more gaps when he exposed that groups shapes can actually give you some indication of where you are in relation to the tune window. For years I had been drawing little pictures of my group shapes in my records and now they made sense. Once again another light came on.

As a rifle goes through its complete tune window the group shapes change in a fairly predictable sequence. Paraphrasing what I remember from Jack's description, here they are...

1. Completely out of tune
   The group is a blob, bullets disperse seeming in all directions by random amounts, some seeming to back into conditions.
   
2. Vertical Stringing
   Whenever I see this, I know that I need to bump the bullet speed. A extra click of powder usually works, as does rotating the tuner toward the chamber about .002"-.003", or seating the bullet a couple thou deeper (my old model).
   
3. Horizontal Stringing with readable sensitivity to winds
   This is the hardest one because it ranges from too wide for the condition (wind sensitive) to just simply a missed letup or pickup. It depends a lot on wind direction, and this portion of the window is very narrow. If it took two clicks on the powder measure to get from #2 to here, it is only a half click to cross this condition to the next. I think this is why we ignore it too often when talk about shot dispersion. These days I test this by moving the tuner another .001" toward the chamber.
   
4. Tighten to a small knot that we all love
   This is the tune that Tony describes as perfect. Not necessarily tiny, but very forgiving of slight mis-reads in conditions. A nice round group that actually represents the true dispersion pattern inherent to the barrel. Shoot it!
   
5. Tighten further to a zero that we believe is perfection.
   Just passed the good group into the fantastic group... tiny bullet hole that doesn't grow. This is actually a scary ledge and while we love 'em, this group is an indicator that you are pushing the envelope. A two degree change in temperature and you get...
   
6. Shots spit into two different holes
   This is the classic 2-n-3 (or 4-n-1) that often measures a .350 just after someone shoots a zero on the previous target. If I see this on the sighter, I usually crank the tuner out toward the muzzle from .002"-.005" and test again. This one is tricky to test because the errant shot might not blow out until the 3rd, 4th, or 5th shot in the group after the first few went in a dot. I've been lulled into shooting these on my record targets several times, even when I knew I should be a-changin' something.
   
7. Completely out of tune again. (Ala #1)
   Another notch up and you're into the next node. Lather. Rinse. Repeat.

Mentally, I visualize this as a progression of the bullet's movement that is managed by controlling initiation pressure and whose outcome is derived from the ultimate flicking from the muzzle. Thus vertical stringing is explained one particular region in the cantilevered beam analysis (vertically rising/falling motion?) but so are the other group shapes as they fall in other portions of stacked torsion and longitudinal vibrations.

 

[CubCouper]

Gads, I just read that crap I wrote. I'm watching too much TV this winter! Argh.

Thank God the shooting season will be underway soon.

Rod

 

[BlackwellADB]

MV speed ???

Rod,

Very informative info, THANKS !!!!

Have you done any graphs on the upper node speed for 6PPC, 3400 to 3475 FPS MV ????

I always shoot faster than 3150 to 3250 FPS.

Al

 

[CubCouper]

I have not. I haven't had much success in the upper nodes, but then I rarely try them either. At 4000-7000 feet elevation (the ranges I normally shoot on) the pressures are just enough different that I get no brass life up there and the no notable accuracy gain to justify working it. After looking at the chart below, I think that I'm causing my own problems by trying to shoot the upper node with the bullet seated too far out.

Here is a table of the initiation pressure vs peak pressure and MV from Quickload -- using the 29.6gr of N133, 65 gr BT bullet, 21" barrel. As you can see, the peak pressures get very high quickly if the bullet has any resistance to starting at all.

Up in this realm, I'm not sure I would trust any computer generated chart to tell me much. There are literally hundreds of formulas with various error factors that get stacked into these numbers and running any cartridge out there on the ragged edge of this performance window requires certain safety precautions and in-depth knowledge of the gun, loading equipment, etc. This information is primary useful for illustrating potential causes of the results we already see.

Initiation Pressure	Peak Pressure	MV
3600	59945	3270
5600	62963	3292
7600	65780	3309
9600	68660	3327
11600	71223	3340
13600	73694	3351

Rod

 

[mks]

Rod,
That is a thorough description of the stages of tuning. Very nice. I had not noticed that the flyer (e.g., 4 and 1) group was one of the stages. This type of group has always been a problem for me. I will have to see this summer if the tuner can manage it.

Some numbers hot off the back of the envelope: To push air at a constant 3000 fps through a 2' long tube of 0.243" diameter takes about 50 psi. This overestimates the actual pressure, because the bullet is moving slower for most of its travel through the barrel. You can play around with the numbers here: http://www.engineeringtoolbox.com/pressure-drop-compressed-air-pipes-d_852.html

To accelerate a 2' column of air to 3000 fps in 1.5 ms takes about 67 psi. This is estimated by multiplying air density by barrel length and by acceleration, which is estimated by muzzle velocity divided by bullet exit time.

Together the total is less than 120 psi. I don't know where Long got his 2800 psi number. It looks more than 20 times too large to me.

0.0005" is the increase in bore diameter that occurs when the chamber pressure reaches 60-70 kpsi. This does not mean that it produces a wave traveling along the barrel of the same dimension. On the contrary, the amplitude of the traveling wave depends on a balance between the momentum of the steel moving radially outward and the stiffness of the steel resisting the motion. The actual wave is very, very small.

Cheers,
Keith

 

[CubCouper]

The 2800psi number was my own... not Long's. I got it from a couple hydraulic cylinder calculators doing what I thought was the same thing: force against a .243" piston for 21". I could believe your number equally well since fluid dynamics is definitely not my wheelhouse. To me, the key is that the bullet is pushing a column of air out into the ambient surrounding. A variation in the density of the ambient surroundings appears to have an effect on tune and it would an interesting exercise to see if the scale of changes match the measurable effect that we observe at either 65 psi or at 2800 psi.

Rod

 

[mks]

To me, the key is that the bullet is pushing a column of air out into the ambient surrounding. A variation in the density of the ambient surroundings appears to have an effect on tune and it would an interesting exercise to see if the scale of changes match the measurable effect that we observe at either 65 psi or at 2800 psi.
Rod

Rod,
The weight of the air in the barrel is about 0.34 grains if it is completely dry. Compared to the weight of the bullet, that is pretty small, on the order of the variation in weight of the bullet itself. At 100% relative humidity, the weight goes down to about 0.11 grains. Even though this change is large, it is still small compared to the bullet. These changes translate to changes in the pressure required to push the air out of the barrel, dropping it from about 120 psi to around 40 psi. Both pressures are so small compared to the maximum chamber pressure of 60-70kpsi that they seem down in the noise. If humidity does affect tune, and I know several who think it does, then it seems unlikely that it is by internal ballistics.

On the other hand, less dense air does affect bullet drop, so there is a direct and probably more substantial effect on vertical tune by external ballistics. By comparison, a 50F increase in temperature only drops air density by 10%. I used to be skeptical about the effect of humidity, but I am coming around (thanks, Gene Beggs )

Keith

 

[tim in tx]

Mike

I'd like to dispel the notion that tuners only affect vertical. That's not the case with any tuner, on any gun, that I've tested them on. Yes, because of gravity and the fact that a mass is attached to the muzzle end of the barrel, the effect is vertically biased...but they certainly do more than just vertical. In use, if or when the gun just begins to go out of tune, it will show up as mostly vertical. When completely out of tune though, IME, they all create big round groups. It only stands to reason that this would be the case and has proven out to be that way in all of my time with many different tuners. We must remember that all of the drawings and FEA movies fail to show all muzzle movement and directions...hence the big round groups when way out of tune. The only explanation I can offer for this notion that they can only affect vertical is two fold. One being misinformation..likely due to making adjustments that are too big to see the groups form and change shape at small enough intervals to see all of what happens to the groups incrementally.

great thread ,I think it is the way you test that determines what you see ,using large [40fps] differing velocities or powder charges will help you map out the movements of the barrel more so then one powder charge,Gene saw just what i am talking about in the tunnel , one reason I think is that bullet depth is worked out before any tuner test are done . I think that is why we have differing beliefs on this matter .But hey that is ok we report what we see and can only believe what we see on target and that is completely understandable but with very close velocities and exit timing you are seeing other things causing random dispersion and the tuner just changes the shape around ,if it was the tuner making big round groups then I think velocity related to poi would give you a better picture of what a tuner does and what I have seen on every gun i have tested for 7-8 years being mostly all vertical. just food for thought.

Tim in tx

 

[mwezell]

I'd like to dispel the notion that tuners only affect vertical. That's not the case with any tuner, on any gun, that I've tested them on. Yes, because of gravity and the fact that a mass is attached to the muzzle end of the barrel, the effect is vertically biased...but they certainly do more than just vertical. In use, if or when the gun just begins to go out of tune, it will show up as mostly vertical. When completely out of tune though, IME, they all create big round groups. It only stands to reason that this would be the case and has proven out to be that way in all of my time with many different tuners. We must remember that all of the drawings and FEA movies fail to show all muzzle movement and directions...hence the big round groups when way out of tune. The only explanation I can offer for this notion that they can only affect vertical is two fold. One being misinformation..likely due to making adjustments that are too big to see the groups form and change shape at small enough intervals to see all of what happens to the groups incrementally.

great thread ,I think it is the way you test that determines what you see ,using large [40fps] differing velocities or powder charges will help you map out the movements of the barrel more so then one powder charge,Gene saw just what i am talking about in the tunnel , one reason I think is that bullet depth is worked out before any tuner test are done . I think that is why we have differing beliefs on this matter .But hey that is ok we report what we see and can only believe what we see on target and that is completely understandable but with very close velocities and exit timing you are seeing other things causing random dispersion and the tuner just changes the shape around ,if it was the tuner making big round groups then I think velocity related to poi would give you a better picture of what a tuner does and what I have seen on every gun i have tested for 7-8 years being mostly all vertical. just food for thought.

Tim in tx

Yes Tim, I agree that it's a good thread and that we don't have to agree on everything. The important thing is that we are here talking about what we find.. In the big scheme of things, whether or not tuners affect only vertical isn't nearly as important as simply being able to make a tuner work for you, consistently.

I'll be posting some test targets when I get time, soon I hope. Time is something that has been all too fleeting and hard to come by of late, though. If the weather cooperates, maybe I can send a few down range next week. I have more important things to do Saturday, as our first match at Gallatin is then. It's been too long since I shot with my good friends there and I'm looking forward to it. I just hope I can shake the cobwebs of a couple of years of too little trigger time, and shoot respectably well. I bet I have a good time either way.

 

[Tony C]

I'll say it one more time, these super light tuners can most times "tune for the moment" but do not have he capacity to permanently tune a barrel into its optimum condition.

Jerry and Others

What do you believe is the ideal weight for a CF tuner on an average BR rifle barrel, and what percentage of the weight needs to be movable or adjustable. (Let's assume that making weight is not an issue up to 13.5 pounds, while keeping the rifle balanced i.e. not muzzle heavy?)