Bart B posted this on another thread

W

William T. Watts

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"Considering all the issues with cartridges and all the compromises one has to make choosing one, the one that gives long barrel life, excellent accuracy, usability in all types of competition, ease of reloading, availability of good components and reloading tools, the .308 Winchester is best. A Kreiger 'tight' 26" 4 groove barrel with a 1:12 twist, .3075" groove and .298" bore will handle 168 to 190 grain bullets with excellent accuracy to 1000 yards."
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I have a Scoot A Duff bore gauge for .308 barrels, I have checked dozens of barrels with the gauge and found few barrels that measure .300" at the muzzle, the largest was .3029. If the lands height of this barrel were .004" it means the barrel groove diameter was .3109", no way will this barrel group boat tail bullets, it may group a heavy flat base if the base is upset enough to seal the lands and grooves. The worst I've owned was a Savage, the rifle shot heavy boat tail bullets 180gr and up and flat base 165gr and up decently but anything lighter all over the paper. I contacted Savage about the problem and they didn't contact me until the following year wanting to know how the rifle was shooting, apparently they thought the rifle had been rebarreled. I shipped the rifle back and "supposedly" they replaced the barrel, when the rifle was returned I checked the bore and it measured "exactly" the same as when I shipped it to the factory. What I believe is no matter how bad a barrel is it usually is installed and shipped and we as consumers buy it and spend endless hours and money trying to find something it will group decently. I have one 30/06 rifle (Ruger Hawkeye) that has an ideal groove diameter .2998" that shoots lights out with 150gr and heavier bullets doesn't matter if they are flat base or boat tail. This is a subject rarely talked about but a major problem why many rifles will not group well.
 
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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.
 
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Bart I'm a life long Rifleman that begin in 1962 at age 20, until I enlisted in the Army I had never handle a rifle nor shot one. We were issued M-1 Garands that had been in long term storage and were our rifles for basic training. Cleaning and subsequent disassemble and assembly when we threw all the parts into a pile and we had to put together a whole rifle and subsequently fire it, shocking but it worked. Fast forward to Colorado while attending Gunsmithing school (TSJC) (graduate in 2001) I picked up a NATO (Danish) Garand with a Var barrel which was shot out (.3021") and replaced it with a Springfield barrel (.3007") simple removal and replace wonder of all wonders it head spaced within tolerance. At this point I became aware of the groove and bore differences in the same caliber in the different manufactures. That's when I connected bore dimensions to the ability of some rifles to shoot Boat tail bullets or another way to say some rifles preferring Flat base bullets. I made a trip to a Cabelas store in Arkansas to buy a Portugal assembled model 70 Winchester, they had two of them in the caliber I was interested (30/06) after checking the bore of the rifles (both well over (.301+) I decided not to purchase either rifle. I pointed out to the sales person the flaws I found in the rifles, while I do not know if they were shipped back to the distributor I suspect they were from the tone of the managers interaction with me.. I had never seen this subject brought up before and I do think this is something people should be aware of!! William
 
Interesting stuff guys. I am really enjoying reading all of your posts and learning about things I had no idea even existed!! thanks for all the great info. keep it coming!!!
 
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