What a difference a few 10th's of a grain makes..

[asmo]

I am working on developing a load for both my and my friends AR platform DM/SPR guns.. Based on a bunch of stuff I chose to start with IMR XBR 8208 powder under Nosler Custom Competition 69gn - with CCI #400's. Again this was a start and will play from here. On Dec 21st I went to the range with my rifle and 10 groups of test loads (5 shots each) ranging from 21.0gn up to 22.8gn of powder. Based on those groups, we worked up some loads between 21.1gn and 21.9gn and ran them through both of our guns to see if we could find a sweet spot that would match both.

Below is the results of the follow-on testing for both guns. Each of the weights had 10 shots from each rifle (20 shots total). All of the brass was once fired LC '02 - shot from different guns (our M4s). All of the brass was trimmed to 1.750" on a Wilson Micrometer Hand Trimmer and checked with our hand-held calipers. Each weight, for each shooter, also had the brass weighed and sorted into .2gn batches.

Rifle 1: Bushmaster A3 type HBAR with 24" 1:9 fluted barrel - chromed lined.
Rifle 2: Colt A3 HBAR Elite with 24" 1:9 crowned barrel - stainless steel.

Both rifles have between 3-5k rounds through their barrels - and have been run hard and put away wet more times than I can count.

We shot at 50m because the wind was gusting between 12-18mph and we didn't want to have to deal with wind calls while trying to group. All of these shots were taken over a 2 hour period on 12/31/2013 at GGC - with temps around 36-38F.

Here are the results:

Rifle/Shooter 1:




Rifle/Shooter 2:




In a couple of the groups there is an un-recorded shot. Those were specifically called shots where the shooter knew he yanked/flubbed it, and immediately called it to the spotter.

I am thinking of going with 21.7gn for the load - knowing that in the future we wont be sorting brass like the above, nor will we be as uber-particular about the trim/weights.

Thoughts?

[yardsells]

I'm for it.

[Danimal]

I had some targets like that a while back where there were a couple grain weights that all tightened down like that. I walked the zero distance back to 200 yards and ran a test group of the same loads and it was easier to spot the best load. Walking yours back to 100 yards or more might be a good idea because a little bit of distance can make a big difference. Since you are making a load for both guns, I would say that 21.7 looks pretty good for both shooters and would probably be a great shooting round even without more testing. But who can complain about another day of shootin?

[TRnCO]

I agree with Danimal, groups at a longer distance will show a truer accurate load, and I'd skip trying to shoot "groups" when looking for a load any time there's more than a couple mph of wind, unless it's a true head or tail wind.
You didn't mention what scopes you're running. If you've got low powered scopes, than shooting groups at longer range doesn't really make much since either. SO, disregard my comment of shooting longer distance if you're running low powered scopes.

[asmo]

We shot at 50m because the wind was gusting between 12-18mph and we didn't want to have to deal with wind calls while trying to group.

The whole point of posting this up was to a) let the other shooter see the numbers and b) get a consensus about the extrapolation of data.

Yes shooting at longer distances will show the increased separation in trajectories - but some times you can't do that. Hence why the numbers are there, to do the math.

[Danimal]

I only recommended that because shooter 1 had several groups that were good, and for shooter 2 21.7 was not the tightest group. It might be shooter #1 has a tighter group at 21.6 at 100 yards, which would correspond better than 21.7 to shooter 2, and provide a more accurate load. Really for true consistency I would have a single shooter firing both guns. If you both fire both weapons, you will have a bigger pool of information for the given loads and you will take the shooter equation out for finding the true best load for BOTH guns.

Really all things considered though, 21.7 looks like a 1MOA group in wind for shooter 1, and a 2MOA in wind for shooter two which is better than most factory ammo so I am sure that you will be happy with it in general. If you did get a nice calm day and 100 yards to stretch it out I would give it another shot with each shooter shooting both guns.

[spqrzilla]

A single group is really not all that informative statistically speaking. I don't know that that series really tells you as much as you think it does.

[USMC88-93]

What program are you using to do those group plots.

[<MADDOG>]

What program are you using to do those group plots.

http://www.ontargetshooting.com/