commercial bullet diameter

[gb0399]

I was watching a video on slugging rifle barrels to determine true bore diameter. This got me thinking, are all commercial loads of the same caliber to the same diameter down to the .001 of an inch? For example.. is every .45 ACP round exactly .450 or are some .449 and some .451?

 

[ford8nr]

Actually .45 Auto are .452 ... Go figure

 

[Bart B.]

Caliber designations for barrels and bullets are mostly general. What their actual diameters are is something else. And sometimes confusing; 35 caliber bullets are used in 9mm Lugers, .358 Win, 350 Rem Mag, .380 ACP, .357 Mag and .38 Spcl.

Barrels are slugged to get their groove diameters. That's what their bullet diameters are close to. I've slugged several commercial 30 caliber barrels ranging from .3069" to .3091". SAAMI specs for them are typically .308" +.002.

30 caliber bullets I've mic'd ranged from .3071" to .3092". SAAMI says their specs are .3090" down to .3060".

"Caliber" originally meant the barrels' bore diameter in even hundredths of an inch; in the USA. It was based on the diameter of the barrel's bore as first drilled. Rifling the barrel added a few thousandths inch.

 

[Sam1911]

Yeah, the answer isn't perfectly simple. Most cast lead bullets are cast on the larger side and jacketed will be a bit smaller. (.430-.431" for .44 Mags for example, while jacketed bullets will be made .429".) But if your particular gun is bored larger than normal you can order oddball sizes (in cast bullets, at least).

Most commercial ammo is jacketed and, as Bart says, you do see a little variation in sizes, though jacketed ammo tends to be pretty forgiving about that as you don't have the leading issue to worry about.

 

[Vern Humphrey]

First of all, most guns have two barrel diameters.

The first is bore diameter -- that's the size of the hole drilled in the barrel.

The second is groove diameter. That's the distance from the bottom of one groove, across the center of the bore to the bottom of the opposite groove.

For example, the bore diameter of a .30 caliber barrel is nominally 0.3000 inches. The grooves are nominally 0.04 inches deep. So the groove diameter would be 0.308 inches.

Some guns are named for bore diameter, others for groove diameter. A good example is the .30-06 versus the .308 Winchester. Both are .30 caliber, but one is named for bore diameter, the other for groove diameter.

Then there are tolerances -- typically a gun barrel is held to about 0.001". So barrels of identical calibers can vary a bit.

And finally, there are names which may or may not have any relationship to bore diameter. For example, the .38 Special has a nominal groove diameter of .357". The .380 ACP has a nominal groove diameter of .355". But the .38 Winchester Central Fire (.38-40) is closer to .40 caliber than to .38.