Example of M1A ammo sensitivity (3 handloads compared)

If you ran the same test several times I would bet you would get a different result every time.

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I use 10-shot groups and shoot only 5 rds per target in a “round robin” process to mitigate barrel heating issues. (10-shot groups are shot in two 5-rd cycles). IMO, 10-shot groups are much more indicative than 5-shot groups. It’s time intensive though.

Since my chrono and accuracy testing 2018 of this load, I have repeated it three times (as I have purchased different lots of I-4064 over the years and I assume a very small burn rate variance could occur). My results have been consistent with groups enlarging by almost double with 0.1 grain variation in either direction. I only use my most accurate rifles with 10x, 16x or 20x optics for this testing.

Naturally I found the results surprising, but three different match M1A rifles with 2 medium and 1 heavy profile Krieger barrel have shown this pattern with this pet load. It’s not a fluke based on testing.

The most surprising element of empirical testing was the chrono results, which I did after developing this load:
October 19, 2018.
M24R with 24” barrel at 100 yards.

Avg velocity: 2658 fps (5 rds)
Standard Deviation: 3 fps(!!!)
Extreme Spread: 6 fps(!!!)
5-shot group was about 0.45 to 0.5 MOA (again, this was a bolt-action M24R sniper rifle).

Same exact load on that same day with same powder lot but with Federal Gold Medal match primer:
Avg velocity: 2638 fps
SD: 10 fps
ES: 29 fps
The group was about 0.75 to 0.8 MOA.

Bottom-line: I have never seen single digit ES with any other specific load using the CCI#34 primer and I-4064 powder. Unfortunately I didn’t keep the chrono results for 42.4 or 42.6 as they were average as I recall, but the 42.5 results with CCI34 primers I kept - as it was almost magical chrono print-out. (Print-out of 42.8 with the Federal Gold Medal Match primers was runner up, with an avg velocity of 2601 fps and an ES of 23 and SD of 9 fps).

I encourage others to experiment and see what results they get, but this particular pet load is very primer dependent, based on my testing.

WaM14gunner said:

Dont forget the condition of the bore for each group. Did you clean the bore to the same place it was for each group? If not, you really cant say definitively that a tenth grain proved anything. Another aspect is the rifle and shooter.

Put a 20 lb, bench rest bolt rifle in a sled and run the test. Shooting hand held with a semi auto introduces many other factors besides the load.

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The cleaning argument is one for which I have seen many different takes. Subjectively, I notice an issue on the first round out of a cleaned and oiled bore, but never on any followups. I have known state and national champs who white-glove clean their rifles (some of them between yardlines) every outing, as well as some who clean once a month and only drop some oil on parts. As long as the rifle is maintained (no rust, well lubricated), they still manage to shoot fantastic groups.

For most shooting, I'd assume the bore is somewhat dirty unless you are only taking single shots each outing on a clean bore - dirty bores are just a fact of shooting groups. And compared to the overall mass of the propellant - which, granted, is part of the discussion here for group size- the combustion byproducts of .1 gr can't be much of the overall percentage of the fouling. Certainly, we all have seen what a few tenths here or there do to our groups, but I assume it has minimal fouling effect. I've never noticed any difference on the patches when I used to clean between different loads, but can confidently state the oiled bore affected the first round.

^ Note for the above, I assume a lot

Here are your impacts mapped together. 95% of the 30 shots were in a circle with a 1.370 inch diameter. That means that regardless of the load used, mathematically, 95% of the time any of those three loads will produce a 1.3 MOA group. Compared to your smallest group, there isn't enough difference to believe that any one of those loads is going to be all that much better than the others, at least not with that one set of groups. On the other hand, since you've found that the 42.5gr load performed consistently well over the years, it's obvious that that would be the best load to stick with.

Good job Random. I've tested many a load at that range and found that the temperature really played hell with my loads. I would come in the summer when it was 80 degrees shot my groups, come back in December when it was 20 degrees an the groups would be completely different. I wish I was good enough to tell how to find a happy node for the different temps that we encounter during the year.

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Well, luckily for us, back in 2008-09 the US military (at Crane) spent a lot of time and dollars to develop a new sniper grade 7.62 NATO ammo that out performed M118LR. They tested over twenty powders and 15 different projectiles. They also tested for temperature sensitivity from -25F to +165 F. What powder did the best overall? I-4064, which was chosen for the then new Mk 316 Mod 0 Special Ball ammo (aka AB 39). Here's some info from a 2009 presentation and a write-up (the powder was listed in a separate document as I-4064):


Excerpt from online article: "7.62 NATO Long Range Match Cartridges" (Part 1) by Frederick Salberta:

"The result, when it was standardized as the MK316 Mod 0 cartridge, was a Federal modified case (Drawing number 8347636), Federal match primer, Sierra 175 grain Matchking and 41.75 grains of IMR 4064 powder. According to the published sources this load will produce a muzzle velocity of 2640 fps out of a 24 inch barrel. The load as produced by Federal is claimed to produce under 1 MOA out to 1000 yards from an appropriate rifle. With the IMR powder the shift in velocity is only 20 fps from 0 to 100 Deg C, with the Standard deviation in velocity over 40 rounds being 15 fps or less. In contrast the M118LR loaded with RE-15 powder will see a 50 fps rise in velocity for a 50 deg rise in temperature.

The MK316 ammunition is essentially the finest possible mass produced match ammunition, comparable to the hand loads utilized by the various service MTUs. The cost is higher than M118LR, with a government cost of 78 cents per round for the MK 316 Mod 0 rather than 55 cents for the M118LR (2009 prices)."
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...My reading of the military testing is that I-4064 must be a very temperature insensitive powder, hence my interest in developing pet loads around that powder for 308 and 30-06 loads. Again, that's what the military tested and selected, and since I don't shoot in the desert of the Middle East, or up in the sub-zero Artic, I suspect it's good enough for us civilians....