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Old December 31, 2012, 08:02 AM   #36
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 4,444
3/10ths grain spread of extruded powder's history from being metered (not weighed) into .308/7.62 cases in high speed loading machinery:

Best lots of M118 match ammo with IMR4895 tested at 600 yards with many dozens of shots per test group; mean radius about 1.8 inch, extreme spread about 5.5 inches.

Most lots of Federal Gold Medal Match using IMR4064 under Sierra 1689's, properly tested at 600 yards in M1 and M14 match grade semiautos; 4 inch groups at 600 yards.

1992 Palma Match ammo loaded on two Dillon 1050's with IMR4895 under Sierra 155's, 3 inch accuracy at 600 yards, under 10 inch accuracy at 1000.

Regarding charge weight spread vs. muzzle velocity, here's what one can deduce from loading manual data and ballistic software......

A 50 fps spread in muzzle velocity from a .308 Win. case will cause a vertical shot stringing from different amounts of bullet drop at 100 yards of about 1/10th inch, at 200 yards a tiny bit over 2/10ths inch. At 600 yards, it'll be about 4.5 inches, at 1000 yards, about 18 inches.

IMR4064 in .308 Win. cases has about 5 fps of average velocity change for each 1/10th grain change in charge weight. With IMR4895, a 1/10th grain charge weight change has about an 8 fps change in muzzle velocity average.

So, if your muzzle velocity spread is 30 fps with exact charge weights of one of these extruded powders, there's something besides the powder charge weight spread of zero that's causing it. Primers? Case neck tension? Weak firing pin impact? How hard you hold the rifle into your shoulder as you shoot bullets through chronograph screens? Or, how much of that spread's caused by the variables in each exact charge weights amounts of propellants, deterrants, stabilizers, decoppering agents, flash reducers, wear reduction and other additives? There's no way two identical charge weights will produce the same pressure curve shapes and dimensions even if everything else has zero spread in performance.
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Last edited by Bart B.; December 31, 2012 at 09:09 AM.
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