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NorthIDIndependent,
In general, you want to stay with powders that are in the same burn rate range as those used by the military. The military used 4895 for M118 Special Ball and M852 match ammunition that used the 168 Grain Sierra MatchKing bullet. Varget and IMR4064 are slightly slower but still in that range. The only powders you have that are not too much slower than those are CFE223 and BL-(C)2. Neither of these are what I would call 'target shooting' powders, but BL-(C)2 is canister grade WC846 which was developed for 7.62 NATO ball ammunition originally, and the AR10 is designed to be happy with that. Just don't go slower.
Your CFE223 is slightly faster than BL-(C)2. Both are high deterrent concentration spherical powders, and it is recommended that a magnum primer be used with them, even though the 308 is not a magnum case. This is because they are harder to light than stick and flake powders are. CCI reformulated their magnum primers specifically for this kind of powder in 1989. The CCI #34 primer would be my first choice for the AR10, as it is a version of their #250 magnum primer tweaked to meet military sensitivity specifications, which the AR10 is designed for. They are actually a little harder (ignition-wise) than the #250 to mitigate any tendency for a floating firing pin rifle to have a slamfire with them. This isn't a great risk with the Armalite design, but why not match what it was designed to shoot unless you just can't get them.
I don't know any magic recipes for those powders, but because they are slower than 4895 or 4064, they will produce the same muzzle velocity with a bit lower peak pressure. You would, therefore, be good to go matching the velocity of known safe loads of the same bullet made using those faster powders and the magnum primers in any 308 test gun you might have on hand.
The way to get that velocity is to buy a box of
Federal GM308M ammunition. This is Federal's match ammunition using your same Sierra 168-grain MatchKing bullet loaded over IMR4064. Fire two fouling shots, then shoot the rest to get an average velocity over your chronograph. Note that GM308M2 is not the same (uses the 175-grain SMK), so don't get that by mistake. The "2" on the end changes the bullet.
HiBC said:
I know 3 shots is not a group. It will show you if the scope is on.
Not very well. On average, the ability of a three-shot group to show the location of the center of all possible points of impact is statistically about 1.73 times better than its ability to show you average group size. But that still isn't doing either very well. This is based on the standard error (SE) which is the standard deviation (SD) of the radial SD of the size of the group. (Radial SD is the square root of the sum of the squares of the horizontal dispersion SD and the vertical dispersion SD.) It is calculated by taking the group radial SD and dividing by the square root of the number of shots in the group. 1.73 is the square root of 3, hence the factor of 1.73 in precision.
Here's an example. I used Excel's random number generator to make a file that simulates shot bivariate distribution modeled after Bryan Litz's simulator. I had it "fire" 1000 rounds in zero wind with perfect sight zero (the yellow-orange cross in the center). It's a solidly 2 moa "gun", but this all scales, meaning that if we change the 2's on the horizontal and vertical axes to 1 and call the biggest ring 1.5 moa and the next one 1 moa and the smallest one 0.5 moa, we would have what a 1 moa gun would do.
Next to the 1000 round target, separated onto targets or their own, are the first three groups of 3 out of that 1000 (the first 9 shots) with all the other shots cleared away. The green crosses are the mean center of each group of three, so if each of the three was used to "correct" the already-perfectly-zeroed sight, the distance and direction from each green cross to the yellow-orange center cross is the size of the error that "correction" would introduce, moving point of impact that same amount to the opposite side of the center cross.
With the exception of the third target, the green crosses are closer to the yellow-orange zero than the groups are wide. That third one just shows this is random and the green being closer than the groups are wide is only true on average, but not necessarily so in every case. The bottom line is, as far as revealing true zero goes, three shots just can't be counted on for critical zeroing.
