Not true at all. From SAAMI:Factory rifle builders and ammunition manufacturers have to follow the Saami specs
American National Standard Voluntary Industry Performance Standards for Pressure and Velocity of Centerfire Rifle Ammunition for the Use of Commercial Manufacturers
Members volunteer to subscribe to SAAMI standards, but there many who don't subscribe even when they're members. Velocity and pressure test barrels, for example. There are no SAAMI standards, that, if not followed, are a criminal offence.
For example, SAAMI specs for 308 Win:
*barrel twist is 1:12 inches. Factory barrel twists range from 1:10 to 1:13.
*308 Win test barrel length specs is 24 inches; factory barrels go from 16 to 30 inches.
*barrel bore and groove diameter specs, have a .002" tolerance; bullets have 003". Bore cross section area spec is .0736 square inch minimum. Imagine the peak pressure difference shooting a .309" diameter bullet in a .306" groove diameter barrel and a .306" diameter bullet in a .309" groove diameter barrel. Both bullets' loads had the same average peak pressure in a SAAMI spec test barrel with .3000" bore and .3080" groove diameters. To say nothing about chamber differences or bullet extraction force.
SAAMI has a glossary to standardize terminology to prevent confusion in communications. Some members subscribing to SAAMI standards use 'head space' to define the space between the breech face and case head when the round is forward against its stop in the chamber. SAAMI says that's 'head clearance.'
It's my opinion that because most folks believe 30 caliber magnums need 1:10" twists is because they started out that way over a hundred years ago. Same for 30-06 barrels that started with the 30-03's 200 grain bullets. Doesn't matter that competitive shooters got better accuracy with slower 1:12" twists in 30 caliber magnums shooting 190 to 210 grain bullets. Note the 308 Win and 7.62 NATO ammo was spec'd with 1:12" twists shooting 150 grain bullets 100 fps slower than the 30-06 did. I had a 9 pound scoped 300 Win Mag sporter with a 1:13" twist 26" barrel that shot 180's into MOA at 1000 yards.
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