The Firing Line Forums - View Single Post - Results: Shooting 180 grain bullets in a 1:12" 308 Win barrel

Bart B. · December 27, 2012, 08:03 AM

For what it's worth, when the .308 Win. was "the" cartridge for NRA com;petition, the two 190-gr. loads that won the vast majority of the matches and set virtually all the records at 600 yards and further was Sierra's 190 HPMK. . .

. . .for bolt guns with custom match barrels, full length sized Winchester case, Rem, Federal, RWS or Win. primer, 42 grains of IMR4064 and the bullet seated to gently seat into the lands.

. . .for service rifles, new LC M118 primed case, 44 grains of IMR4320 and bullet seated about .020" short of the lands.

Most folks shooting Sierra's HPMK bullets in factory match rifles didn't get best accuracy. Those factory barrels were typically too big in groove diameter. Sierra's .3082" HPMK match bullets shot best when the barrel's groove diameter was at least a few 10/000ths smaller; .3079" or smaller. If they used Lapua's 185-gr. .3092" diameter FMJ rebated base match bullets, they shot very well.

I think one should ignore the smallest groups shot when testing a load for accuracy. The smallest ones happen only when all the variables cancel each other out. If you want to see how all the variables contribute to inaccuracy, then use the largest groups for a given load to judge them. Those big ones are the results of all the variables adding up in the same direction. And they show you how much you'll typically miss your point of aim. Any load will sometimes shoot very tiny groups all about the same size, but only a few loads will shoot their largest groups the smallest.

December 27, 2012, 08:03 AM	#18
Bart B. Senior Member Join Date: February 15, 2009 Posts: 8,927	For what it's worth, when the .308 Win. was "the" cartridge for NRA com;petition, the two 190-gr. loads that won the vast majority of the matches and set virtually all the records at 600 yards and further was Sierra's 190 HPMK. . . . . .for bolt guns with custom match barrels, full length sized Winchester case, Rem, Federal, RWS or Win. primer, 42 grains of IMR4064 and the bullet seated to gently seat into the lands. . . .for service rifles, new LC M118 primed case, 44 grains of IMR4320 and bullet seated about .020" short of the lands. Most folks shooting Sierra's HPMK bullets in factory match rifles didn't get best accuracy. Those factory barrels were typically too big in groove diameter. Sierra's .3082" HPMK match bullets shot best when the barrel's groove diameter was at least a few 10/000ths smaller; .3079" or smaller. If they used Lapua's 185-gr. .3092" diameter FMJ rebated base match bullets, they shot very well. I think one should ignore the smallest groups shot when testing a load for accuracy. The smallest ones happen only when all the variables cancel each other out. If you want to see how all the variables contribute to inaccuracy, then use the largest groups for a given load to judge them. Those big ones are the results of all the variables adding up in the same direction. And they show you how much you'll typically miss your point of aim. Any load will sometimes shoot very tiny groups all about the same size, but only a few loads will shoot their largest groups the smallest. Last edited by Bart B.; December 27, 2012 at 08:13 AM.