A velocity question for you smart guys

J

jlcprec

New member
In a 6 PPC benchrest rifle, custom action, Krieger barrel, tight neck, etc. what kind of extreme velocity spread would you expect to see using the same case for each shot? Powder charges weighed to .00 resolution on a gemtech scale, 68 grain bullets, speed in the 3360 fps range. I'm using a Lab Radar for the test.

Jim
 
well a gemtech 250 is only 0.02 not .00
the tension may change with each shot depending on the age of the case.
i have HEARD br guys say that have seen 40 on .1 groups
shooter skill, rifle quality
still lots of variables.
 
jlcprec said:
In a 6 PPC benchrest rifle, custom action, Krieger barrel, tight neck, etc. what kind of extreme velocity spread would you expect to see using the same case for each shot? Powder charges weighed to .00 resolution on a gemtech scale, 68 grain bullets, speed in the 3360 fps range. I'm using a Lab Radar for the test.

Jim
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Jim, this is something I always wondered about myself with first the 6ppc and later with my own 6mmBeggs cartridge. I think it has something to do with the relatively small case capacities. The long range BR shooters report single digit extreme spreads with their typical 1000 yard rounds while I have never been able to achieve anything close to that.

In the tunnel where most of my testing has been done in the past fifteen years, I use an Oehler 35 chrono and most of my extreme spreads show 35 to 40 fps variation in muzzle velocity using N133. When I started using LT-32 my ES was usually cut in half averaging about 20 to 24 fps spread. This in spite of using the same case loaded over and over as you suggested and carefully weighing each charge on an Ohaus labratory scale. Some shooters think they are getting much smaller ES when weighing and using a Chargemaster when if they were to check it out carefully with a good chrono they would find they are wasting their time.

I simply 'throw' all my powder charges directly into the case from a Jones measure. My chronographed extreme spreads are unchanged from those using the same case twelve or fifteen times and weighing every charge to the nearest gazillionth. The concern over plus or minus .1 grain variations in thrown charges or even the occasional .2 grain spread is nonsense. I quit worrying about it and rely on my tuner to keep the rifle in tune taking advantage of what is known as "Positive Compensation." My performance at the Cactus this year proves my theories are correct.

I'm a pretty slow learner and it has taken me about thirty years to finally learn what works and what doesn't. Some things make a difference and some don't. Weighing charges for use in shortrange BR is a waste of time. Buy a good measure, practice with it and throw your Chargemaster away.

Gene Beggs
 
Hi Jim, At the Cactus this year I tested four shots using a Labradar with a load of 30.2 grs of V-133, and I recall getting a 6 fps velocity spread with Hottenstein bullets.
 
I don't think it matters much but that's just me. However, something matters that I don't know about as the aggs keep getting smaller and teen aggs are more prevalent
 
This comes up periodically. The question whether velocity spread is important in 100/200/300 yard Benchrest.

I periodically shoot over a chronograph. It is not uncommon to shoot groups that measure .150 or better that have velocity spreads as much as 30fps.
 
jlcprec said:
In a 6 PPC benchrest rifle, custom action, Krieger barrel, tight neck, etc. what kind of extreme velocity spread would you expect to see using the same case for each shot? Powder charges weighed to .00 resolution on a gemtech scale, 68 grain bullets, speed in the 3360 fps range. I'm using a Lab Radar for the test.

Jim
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Jim,

It depends! I’ve had a labradar on the bench for nearly every shot for the last three years. That includes practice and competition! So I’ve learned several things. But the more your barrel cools between shots it will slow velocity down (to a certain extent). So one shot at a time may not be the best way to get a low ES.

PS
Low ES are easy to attain but don’t mean squat for 100/200 yard Benchrest.
Bart
 
I know some powders are much better than others at low ES, but that doesn't mean they shoot well.

I've wondered if some of it may be ignition problems with the action-trigger.

With some lots of N133, my thrown charges in different cases give better ES than the weighed ones using the same case.

I've only tried this with one rifle, I'm going to try again with a different flavor some other time. I know the es doesn't necessarily correlate to group size, but maybe it can indicate other problems.
 
jlcprec said:
I know some powders are much better than others at low ES, but that doesn't mean they shoot well.

I've wondered if some of it may be ignition problems with the action-trigger.

With some lots of N133, my thrown charges in different cases give better ES than the weighed ones using the same case.

I've only tried this with one rifle, I'm going to try again with a different flavor some other time. I know the es doesn't necessarily correlate to group size, but maybe it can indicate other problems.
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Exactly right.
 
Like Bart, I shoot next to a Lab Radar. But I could care less about ES and SD for initial tuning. I've had LT-30 & LT-32 loads which shot dots but gave ES's of 30+ fps. Similarly, I've had tight spreads that were clearly between nodes (lots of vertical). The Lab Radar is really helpful for staying on a node as conditions change. And if you shoot powders like LT, which have pretty wide nodes, super tight ES and SD aren't something you chase.

-Lee
www.singleactions.com

PS - I only shoot short-range BR. 600 yards and out, I'd care more about ES and SD.
 
While we are talking about velocity, how about this question.

Giving equal agging capability, which would you shoot. A 6PPC shooting 68 grn bullets at 3350/3400 or a 80 grn bullet going 3150?
 
Jackie,

I've tried the 80 grainers, and even if it shot better in the wind, I can't handle the recoil from an 80 grain bullet in an LV.
 
jlcprec said:
Jackie,

I've tried the 80 grainers, and even if it shot better in the wind, I can't handle the recoil from an 80 grain bullet in an LV.
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I shot my HV with the 80 grn Barts at Seymour, decided to chamber a 1-12 Krieger for my LV. I tested it last Sunday. Not a problem at all. It shot pretty darned good, and I didn't even notice the recoil. It's certainly nothing like my HV 30BR.

The biggest problem with the LV is to make weight, I had to settle for a 22 1/2 inch barrel, instead of the 25 1/2 on the HV. The difference is 100 fps.
 
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I agree that small groups can be shot in short range BR with a large ES, just as long as the velocity swing isn't so wide that you get outside of your tune window. I have personally seen single digit ES with weighed charges from the 6PPC, but it is not common. I believe the 30BR would show smaller ES than the 6PPC. Below is a report from my Labradar. This is typical for the 30-30 with weighed charges. While I don't put greater faith in my lowest ES loads, I believe if anything, it tells you things are working together like consistent ignition. This load was a 118gr bullet with 39.3gr of N133 and a large Wolf primer.

Michael
 

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Do the general comments

that ES doesn't matter much in short rang br a function of the short range or that some barrels handle certain spreads better than others...in other words go out of tune with a smaller es range?
 
glp said:
that ES doesn't matter much in short rang br a function of the short range or that some barrels handle certain spreads better than others...in other words go out of tune with a smaller es range?
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Greg,

This mainly has to do with bullet drop. A 6PPC at 100 yards will only see about .05" bullet drop if a bullet that is part of a group is running 40 fps slower. If you look at a 1000 yard rifle shooting a .700 BC bullet at 3000 fps, a bullet that is part of a group will hit about 6.5" below the group if the velocity is down by 40 fps. These are estimates based on ballistic programs.

Michael
 
mturner said:
Greg,

This mainly has to do with bullet drop. A 6PPC at 100 yards will only see about .05" bullet drop if a bullet that is part of a group is running 40 fps slower. If you look at a 1000 yard rifle shooting a .700 BC bullet at 3000 fps, a bullet that is part of a group will hit about 6.5" below the group if the velocity is down by 40 fps. These are estimates based on ballistic programs.

Michael
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So,that .050" could mean the difference between a mid-one and a two at 100yards and .100" at 200 yards....right? To me,40fps is huge. Just my thoughts...…...Dan
 
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gpoldblue said:
So that .050" could mean the difference between a mid-one and a two at 100yards and .100" at 200 yards....right? To me 40fps is huge. Just my thoughts...…...Dan
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Well, no, not really.....

The whole concept of (tuning) a BR rifle is built around the fact that when tuned the rifle self-compensates for the velocity difference.
 
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