Help me choose my next test loads

[blockfort]

My last test results, all groups are 8 shots, done round robin over a 2 hour session. Paper has 1/4" grid.

.308 with 42.5, 43.0, 43.5, 44.0, 44.5 grains IMR4064, 168 SMK, 2.8 OAL



42.5 has the best "grouping", but 43.0 has the smallest Extreme Spread and Standard Deviation. 44.5 is close behind in 3rd place for both ES and SD, and seems like 45 may have shrunk down further. I'd like to test slightly above and below 43.0, but also explore 44.5 and further above. The groups obviously go from high to low, and are more scattered in the middle. Does this represent the rounds coming out at the top and bottom of the barrel's diving-board-like vibration?

I have 50 brass prepped, I was going to keep 5 for sighters/foulers/barrel warm-up which leaves me 45.

My idea at the moment is 5 rds each of:

42.6
42.8
43.0
43.2
43.4

and

44.6
44.8
45.0
45.2

Thoughts, ideas?

[bsumoba]

I think you got it. 42.5 appears to be around or near the barrel's "upwards barrel whip", but might be past it. 44.5 appears to approach the bottom of the barrel whip. Most argue that the load that approaches the top of the barrel's whip is best and that is where you will typically see better ES/SD, at the bottom/top of the barrel whip.

What rifle is this? Factory rifle?

I would load a couple rounds, and load in 1/2 gr increments until you reach pressure. Once you find pressure, do not shoot the rest. Knowing the point where your rifle shows signs of pressure is always good. Once you find the point where the load shows pressure signs, back off a full grain and work up loads from there. This ensures you are in a safe region and if you find a load around there, you will know that you are at the max velocity node for your rifle.

[blockfort]

It's a (used) factory Browning A-Bolt Target.

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[Unretarded]

42 grains did well in a m1a with a 175 grain nosler, but not a great load for max distance.....the numbers looked good and the group was tight.......right around 2600 fps in a 22 inch barrel national match.