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Old January 28, 2015, 11:27 AM   #3
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 6,508
Bill Watts, you're one of the few who are aware of such bore diameter issues. I've got two Garand throat erosion gauges with a .2990" small end. Neither goes in the muzzles of my Hart, Obermeyer or Kreiger 30 caliber match barrels. Your comment about oversize barrels and flat based bullets. Sierra learned in the 1950's that flat based bullets shot most accurate in commercial factory barrels; most were on the larger groove diameter side. Custom match grade barrels with groove diameters smaller than their boattail bullets shot them the most accurate; a little more so than flat based bullets.

I'll add an interesting fact; plus:

The British Commonwealth learned in the 1960's when they quit using their SMLE .303's in long range matches and started using 7.62 NATO barrels and ammo (had to use arsenal ammo, handloads were not allowed), they soon learned that the typical .3070" to .3075" NATO bullet diameters did not shoot all that accurate at long range. Their custom barrel makers made groove diameters in the .3065" range; success soon followed. John Kreiger makes .308 Win. Palma barrels for them with .3065" groove diameters.

When the USN got their first batch of 7.62 NATO chambered barrels to convert Garands, they air gauged all of them for groove diameter tolerances. Those barrels broach rifled after the teeth on the broach had worn down quiet a lot and made .3075" to .3079" groove diameters, they were set aside for match grade conversions. The other barrels gauged .3080" to .3086" groove diameters and used for normal service/combat use; quite normal with Springfield Armory's tool and die shop making the broaches which removed the most metal when new.

I've shot .3092" diameter Lapua match bullets very accurate in Winchester factory .308 Win. match barrels with a .3085" groove diameter. Sierra's with a .3082" bullet diameter never shot all that well in them. No wonder Western Cartridge Company made their 197 and 200 grain match bullets for Winchester 70's at .3088" diameter; they knew what bullet diameter shot best in them.

Military team members shooting match grade service rifles for the 7.62 NATO round sometimes had two rifles. One with a barrel having a groove diameter about .0005" smaller than Sierra's .3082" diameter match bullets. The other was an arsenal match grade barrel with groove diameters about .3083 to .3085" but would shoot the M118 match ammo's 172-gr. FMJBT bullets with .3086" diameter very accuracy but poorly with the smaller Sierra's.

When the military tested long range machine gun bullets for long range accuracy back in the 1920's at Daytona Beach, FL, they quickly learned the 150-gr. .3078" diameter ball bullet was not all that great in machine gun barrels. They went to a larger diameter, 172-gr. FMJBT bullet that did much better. It became the standard "match" bullet for decades but had its crimping cannelure removed later as it was a little more accurate without it as well as no case mouths were needed to be crimped into it.

You'll need stuff you can test no worse than 1/3 MOA at 100 yards or 2/3 MOA at 600 to tell the difference.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master

Last edited by Bart B.; January 28, 2015 at 12:36 PM.
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