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Old February 15, 2006, 12:36 PM   #5
Archie
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Join Date: May 26, 2000
Location: Hastings, Nebrasksa - the Hear
Posts: 2,209
Twycross has it.

In a perfect world, all firearms bores would be made exactly to specifications, as would all bullets.

Bullets are sized to exactly fit the groove diameter of the firearm in question. The lands engrave the outer few thousandths of the bullet or jacket in order to impart spin upon firing.

Of course, this is not a perfect world. Both barrels and bullets are subject to 'tolerances'; that is a '30 caliber' barrel has a groove diameter of .3085" and a land diameter of .308" plus or minus a few ten-thousandths. Actually, modern machining methods produce barrels of superb uniformity; which is not to say the occasional clunker gets through the quality control. Modern bullets are very uniform as well. Most of the major bullet makers produce hunting ammunition superior in accuracy to match bullets of fifty years ago.

Military weapons of the past have varied greatly in specifications. 9x19 and .380 ACP pistols are both 'officially' .356 groove diameter; however, major manufacturers in various countries have produced such handguns with barrels ranging from .354" to .362". And they all shoot the same bullets. Wartime production of ammunition varied as well.

As you might expect, a small bullet in a large barrel produces only marginal accuracy. In addition, if the bullet is hard enough not to obdurate sufficiently, much power is lost in the propellent gases 'blowing by' the bullet. Larger diameter bullets simply 'smoosh down' (that's a technical term) to bore diameter.

Revolvers pose another problem. Not only must bullets fit the barrel, they must first pass through the throat of the respective cylinder. So, for maximum accuracy, cylinder throats should be reamed out to .0001" larger than the bore groove diameter. Sizing revolver bullets is a futile excercise; they get re-sized several times between chambering and the bullet leaving the barrel.

Now; one more cock-eyed thing about bore and bullet diameters. The popular name for a round may or may not have anything to do with the actual size of the bore.

.38 S&W, .38 Special, .357 Magnum, Super .38, .380 ACP, and most all the 9mm something or others are all in the range of .354" to .362". Except for extreme bullet weight differences, the projectiles are pretty much interchangable. I've loaded and shot 90 grain .380 ACP bullets in .357 Magnum cases. I've loaded and shot 158 grain lead round nose .38 Special bullets from a 9x19 autoloader. I've loaded and shot 125 grain JHP .357 Magnum bullets from a .380 ACP (they were too long to fit in the magazine, so I had to single load them).

.303 British is actually .323". .303 Savage is actually .308". 7.7mm Arisaka is the same as .303 British. 8mm Mauser is actually .323", or .312" in the earliest loading. 6.5x55 Swedish Mauser, .256 Newton and .264 Winchester Magnum are all the bore diameter; .256" is the land diameter, .264" is the groove diameter and 6.5mm is the metric equivilent of .256". Then, .257 Roberts and .25-06 are both .257 groove diameter.

Okay, that's more than you asked, but I was on a roll.

Rivers, I do not follow your thought at all?
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Archie

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