Clarifying definitions for Jump, Touch, Off, Jam and Push Back.

adamsgt

adamsgt

Jerry Adams
Trying to get set up for my new to me 30BR. I bought bullets from four custom bullet makers. I started out by making a "jam" cartridge for each bullet and measuring them to go forward for tuning. I then re-read Boyer's book on his method and am trying to ensure I understand what each term means. I used the jam cartridges to determine a seating die setting for "10 off".That seemed to go well until I tested the settings with other cases. It appeared that the initial jam reading did not agree with subsequent attempts. At this point I decided that doing four bullets at once was kind of idiotic and I better fall back and regroup. That led me back to Tony's book and trying to understand the relationship between the terms.

From what I glean from the book, jump is the distance between "touch" and the ogive of the bullet. Touch is the point at which the bullet first engages the lands. Off is the distance between jam and touch. Jam is the point at which the bullet is fully engaged in the lands. I was unable to replicate any "push back" so I'm not sure what it means of how to use it.

Here's some data from one set of cases and bullets. Jam measurement: 1.618, 1.6215, 1.623. So there's about 5 thou difference between the first and third case and bullet. I measured the bullet base to ogive and had the same reading (.162) for the first and third while the middle bullet had (.164). Some attempts with the other bullets had larger differences up to 10 thou. So while 5 thou doesn't sound like a large value it's significant when you're talking about changing seating depth 3-5 thou to tune. In his book, Tony uses three thou increments to set up his test. So, how do you know where you're starting from if the jam measurements aren't consistent.

Was planning on loading a test for one bullet and going to the range tomorrow but now I'm not sure what I want to set up. When I put the holes in the paper I want to ensure that I understand why they're there. No point in wasting powder and bullets if you don't understand the results.
 
The dimension that we call jam depends on how hard the brass is, even though cases have necks of the same thickness, and the same bushing is used for all, the point that a bullet will be pushed back to, as a test round is chambered, will vary with how hard the brass is, and the interior finishes of the case necks. IMO for the latter reason, it is best to use a case that has been fired a few times, so that its neck ID fouling will have reached a stable condition. Did you notice a difference in the force it took to seat bullets in your four cases? If your bullets are uniform, I believe that differences in bullet seating force would account for the spread in jam that you saw. In any case, I would choose the longest of the numbers that you got, and use it as a basis for your further seating depth experiments. That way, if you are seating shorter than that, none of your loads will be at jam, assuming that the cases that you chose are representative.

I normally just start with one case of the small set (perhaps 15 or so) that I will be working with (I don't preload.) and go with what it give me, making adjustments and shooting test groups. The only tricky part about using jam is when I hear someone quote a number off, or shorter than jam that I have reason to suspect puts them shorter than touch. For seating depths that involve any amount of jump, I would rather start with touch, and work back from there. I have a couple of ways to measure where a bullet is touching, with some practice, I think that the Hornady tool and modified case is the most sensitive, but if I am working up something for a field rifle, I generally get out my Sinclair tool that has an adapter that goes in the back of the action, a rod, and stop collars, which reminds me, I need to get another wood dowel to knock loose bullets when I am using that method. Inserting cleaning rods from the muzzle tends to raise my anxiety level.
 
When I am determining a seating depth, I seat a bullet long in a sized case and finish seating it with the bolt, I call this hard jam. The reason this number is inconsistent is slight variations in neck tension but this is not that important since this is a base number only,so do 1 round at hard jam and measure the seating depth at hard jam and adjust seater to seat the bullet .003 deeper in the case or .003 off of hard jam and try this with a powder charge that gives you 2950-3000 fps. Try 2 rds. at this seating depth and if they don't go into a hole, move .003 deeper into the case and try 2 more. When you get 2 to go in a hole try 3 shot groups. Try a few more steps of .003 on the seating depth until the group tightens up and when the group opens up go back to the depth that shot the best and try moving seating depth .001 in or out to see what shoots the best. Once you determine the seating depth then adjust powder charge up to the 3000-3050 range and see which shoots the best. All of the 30 BR's that I own have shot the best between .005 to .020 th's off of hard jam.
 
I keep it pretty simple. I back the bullet off in .005" increments until I can't feel the bullet seat when I close the bolt. That's when it usually really starts shooting and flyers go away. It also allows you to extract a live round without dumping a case full of powder in your action. Usually, I'll take the arbor press and seater die right up to the line with me. Record the protrusion length of the seater stem for that particular bullet and call it done. The 30BR is pretty easy to tune.
 
Measurement's don't actually mean anything to the rifle. Please don't get confused here - it's really easy.

Make some loaded rounds that the bolt closes on rather difficult...a small degree of too difficult. Then, shoot some of those to see what happens (you don't want to leave it there). Move your seater in to decrease the length of the round and shoot some more...looking at the groups to see what happens. Continue that until you find the right seating depth for that bullet. If you think you need to, measure the seating stem or whatever you need to measure so you can come back to that when you goof up your seater.

If you do it this way, the only direction you'll need to go is one way...and then back to the best maybe. Nobody ever got a rifle to shoot by measuring anything. They simply measured where the rifle shot best once determined.
 
Wilbur said:
Measurement's don't actually mean anything to the rifle. Please don't get confused here - it's really easy.

Make some loaded rounds that the bolt closes on rather difficult...a small degree of too difficult. Then, shoot some of those to see what happens (you don't want to leave it there). Move your seater in to decrease the length of the round and shoot some more...looking at the groups to see what happens. Continue that until you find the right seating depth for that bullet. If you think you need to, measure the seating stem or whatever you need to measure so you can come back to that when you goof up your seater.

If you do it this way, the only direction you'll need to go is one way...and then back to the best maybe. Nobody ever got a rifle to shoot by measuring anything. They simply measured where the rifle shot best once determined.
Click to expand...
Wilbur, that's about as simple and easy to understand explanation as I've ever seen. One suggestion that I'd add to that is once you find the seating depth where your rifle wants to shoot best, use a Davidson type ogive checker and measure a seated round recording that measurement for future use. When you get a new lot of bullets of that same make, adjust your seating depth to the recorded measurement and you'll be pretty close to the original seating depth. I use the Davidson ogive checker all the time now days especially when going from one type of bullet to another type. It seems to save a lot of time. I think Sinclairs still sells the Davidson ogive checker although Alvin has been gone for quite some time now.
 
steve stanley said:
When I am determining a seating depth, I seat a bullet long in a sized case and finish seating it with the bolt, I call this hard jam. The reason this number is inconsistent is slight variations in neck tension but this is not that important since this is a base number only,so do 1 round at hard jam and measure the seating depth at hard jam and adjust seater to seat the bullet .003 deeper in the case or .003 off of hard jam and try this with a powder charge that gives you 2950-3000 fps. Try 2 rds. at this seating depth and if they don't go into a hole, move .003 deeper into the case and try 2 more. When you get 2 to go in a hole try 3 shot groups. Try a few more steps of .003 on the seating depth until the group tightens up and when the group opens up go back to the depth that shot the best and try moving seating depth .001 in or out to see what shoots the best. Once you determine the seating depth then adjust powder charge up to the 3000-3050 range and see which shoots the best. All of the 30 BR's that I own have shot the best between .005 to .020 th's off of hard jam.
Click to expand...

Now this guy gets it.
 
Wow

You guys provided some really good suggestions. I think I again made the error of assumption. That being that I needed to preload before the test. I've done testing and loading at the range for my 6PPC but that was in cooler weather. I've got the tools suggested, except for the optical comparitor suggested by Paul Revere. I've tried to find one on ebay but they always seem to want more than a hundred bucks for one. :p For some reason I didn't connect the Sinclair seating depth tool with "touch" which is what it does measure. I have the tool but, again, for some reason I didn't use it this time. Steve's suggestion reminds me that I need to chronograph the loads.

Yeah, I need to stop, take a couple of deep breaths, pick one bullet and start over while incorporating your suggestions.

Thank you all.
 
Last edited:
Wilbur said:
Measurement's don't actually mean anything to the rifle. Please don't get confused here - it's really easy.

Make some loaded rounds that the bolt closes on rather difficult...a small degree of too difficult. Then, shoot some of those to see what happens (you don't want to leave it there). Move your seater in to decrease the length of the round and shoot some more...looking at the groups to see what happens. Continue that until you find the right seating depth for that bullet. If you think you need to, measure the seating stem or whatever you need to measure so you can come back to that when you goof up your seater.

If you do it this way, the only direction you'll need to go is one way...and then back to the best maybe. Nobody ever got a rifle to shoot by measuring anything. They simply measured where the rifle shot best once determined.
Click to expand...

Thanks Wilbur. As usual I started overthinking it.
 
Mike Bryant said:
Wilbur, that's about as simple and easy to understand explanation as I've ever seen. One suggestion that I'd add to that is once you find the seating depth where your rifle wants to shoot best, use a Davidson type ogive checker and measure a seated round recording that measurement for future use. When you get a new lot of bullets of that same make, adjust your seating depth to the recorded measurement and you'll be pretty close to the original seating depth. I use the Davidson ogive checker all the time now days especially when going from one type of bullet to another type. It seems to save a lot of time. I think Sinclairs still sells the Davidson ogive checker although Alvin has been gone for quite some time now.
Click to expand...

Mike thanks, the .30 Davidson nose piece arrived last Friday and I'm using it. That's the one tool that makes digital calipers useful.
 
If you have worked up a load for a ppc the 30 BR is done the same way, the 30 br is usually a lot less finicky than a ppc. Don't over think it, it is really pretty simple to work up a load. Good shooting! :)
 
adamsgt said:
Mike thanks, the .30 Davidson nose piece arrived last Friday and I'm using it. That's the one tool that makes digital calipers useful.
Click to expand...

A simple sinclair nut will do just as well. Ive even been known to go out and shoot a .25 auto to get a comparator tool as well. Use your imagination.
 
steve stanley said:
When I am determining a seating depth, I seat a bullet long in a sized case and finish seating it with the bolt, I call this hard jam. The reason this number is inconsistent is slight variations in neck tension but this is not that important since this is a base number only,so do 1 round at hard jam and measure the seating depth at hard jam and adjust seater to seat the bullet .003 deeper in the case or .003 off of hard jam and try this with a powder charge that gives you 2950-3000 fps. Try 2 rds. at this seating depth and if they don't go into a hole, move .003 deeper into the case and try 2 more. When you get 2 to go in a hole try 3 shot groups. Try a few more steps of .003 on the seating depth until the group tightens up and when the group opens up go back to the depth that shot the best and try moving seating depth .001 in or out to see what shoots the best. Once you determine the seating depth then adjust powder charge up to the 3000-3050 range and see which shoots the best. All of the 30 BR's that I own have shot the best between .005 to .020 th's off of hard jam.
Click to expand...

OK, Quickload says that 34.1 gr of H4198 behind a Berger .30 115 gr bullet will yield a muzzle velocity of 2954 fps. One of my lots of .30 custom bullets is 115 gr and they are the best looking and consistent of all of them, so that's what I'll start with.

I dropped a 34.1 charge into a primed case and it was about halfway up the neck. Does the velocity and fill sound about right for that bullet and powder? Also, if I increase the charge to 35.0 H4198, Quickload shows a muzzle velocity of 3023 fps and 62205 psi chamber pressure. There is still some room in the neck for the bullet although it may be a slightly compressed load. I have shot the gun at 33.8 gr so I'll start there first to see what I really get as to velocity.
 
Any of those powder charges should work, if you need a little more room for the bullet, slowing your powder drop speed will give it to you. Half way up the neck sounds about right for a normal speed drop.
 
Last edited:
Loaded up three sets of three to get some velocities. Started with 33.8 of H4198 and went up by .3. Seated all the bullets to 3 off jam. Strapped the Magnetospeed to the barrel and got the following results. The 34.1 showed a promising cluster.

33.8 = 2974, 2990, 2999
34.1 = 3006, 3013, 3019
34.4 = 3030, 3041, 3041

Don't know if the 96 degree temp could have affected the velocities. It was hot!

So, Steve, I've got a starting point, thanks for your help.
 
You are welcome Jerry, I don't think it is possible to get enough 4198 in the 30BR to get in trouble, pressure wise.I wouldn't fool with the powder charge much until you find the seating depth, the seating depth is more critical to accuracy. Once you find the right seating depth play with the powder charge to see what the gun wants and you should be good to go. I hope you have a great barrel, they are one of the treasures of life! :)
 
Last edited:
PAUL REVERE said:
lots of powder differ.
there was a thread here about a new guy with a 30br...
go read it...
there were velocity issues in that thread.
as i recall..do not quote me, quickload was too hot///well hotter than expected
Click to expand...

In this case, Quickload seems to be a bit low. It estimated 2954 fps for 34.1 gn while the actual was 3006, 3013, 3019. I'm currently using H4198 that has a 2008 date. I was using it in a 6 Beggs cartridge. I have a jug and a half of that left. I also have a 2012 Jug that I purchased recently. Might be ineresting to see what that gives.
 
If you are weighing charges I don't think you will see a lot of difference, if you are throwing by volume you may need to reset your thrower with the scale possibly when you change lots. I always check my powder thrower with scales when I start a new lot of powder.
 
You must log in or register to reply here.
Back
Top