What Caliber Is Your Cartridge?

What caliber are you shooting? That’s one of the first questions other shooters ask when they meet you at the range or in the hunting fields. But it’s not what they mean.

The “caliber” question is really a “cartridge” question. Caliber is the diameter of the rifle’s bore. Answering it correctly does not describe the ammunition the rifle fires. You can shoot a 30-caliber, but the cartridge could be a .30-30 Win., .30-06 Springfield, .300 Win. Mag., .300 Savage, .300 Wby. Mag., .308 Win., .30 AR Rem., .300 Blackout, .300 Win. Short Mag., .300 Rem. Ultra Mag, .300 Dakota, .300 H&H Mag, .300 Norma Mag, .30-378 Wby. Mag… .

For clarity, it’s important to know the differences between bore caliber, bullet caliber and cartridge.

Bore caliber is the diameter of the bore, the hole in the barrel. If that hole is rifled (spiral grooves cut into it,) some would reference the diameter across the bottom of the rifling grooves. This is why some shooters refer to .30 calibers as .308 calibers. The .308-inch dimension is both the groove-to-groove diameter and the bullet diameter. Bullets have to be slightly wider than bores in order for the rifling to grip them. The bullets are the same diameter as the diameter of the rifling grooves, and that’s why you see rifling marks or grooves cut into bullets after they’ve been fired.

When referring to sporting arms, caliber can be expressed in inches or millimeters. The British and U.S. tradition is inches. Other countries use the metric system. Thus, a .28 caliber in the U.S. would be a 7mm in Europe. A .264 caliber here is a 6.5mm, a .308 caliber is 7.62mm, a .243 is a 6mm and a .224 is 5.56mm.

A .30-06 is a cartridge that includes a .308 caliber bullet that will be fired through a .30 caliber barrel. And once the cartridge is fired, the bullet is gone, the powder is burned, the primer is spent, and the empty case remains.

Cartridge is the proper name for the whole round, which includes primer, brass case, powder, and bullet. The .30-06 cartridge shoots a .308 bullet through a .30-caliber bore.

Some shooters refer to the cartridge as a bullet, which can get real confusing. One gentleman was describing an accident in which his cartridge apparently suffered head separation. That’s when a brass case breaks apart just above the head of the case. Generally the front half sticks in the chamber while the extractor pulls the head out. But this guy described his bullet as breaking in half inside the rifle, which is a whole ‘nother matter. His description wasn’t reaching his audience because he didn’t use the proper nouns to identify the parts.

Real confusion results from all the goofy names given cartridges. They often make no sense. The .30-30 Winchester is a .30 caliber, but the second “30” refers to the original weight of powder in the charge. The 30 in .30-06 also refers to bore diameter, but the -06 is the year it was created. The various .300 magnums are .30 calibers with an extra zero added, perhaps to signify the extra velocity from the magnum dose of powder. The .308 Winchester muddies the water by using bullet caliber instead of bore caliber. The famous .22-250 Rem. is a .250-3000 Savage case necked down to .22 caliber. And so it goes.

Here is a list of popular bore calibers and bullet calibers that fit them:

Bore caliber in inches & millimeters         Bullet caliber in inches

.17 4.3mm                                       .172

.20 5mm                                         .204

.22 5.56mm                                      .224

.24 6mm                                         .243

.25 6.4mm                                       .257

.26 6.5mm                                       .264

.27 6.8mm                                       .277

.28 7mm                                         .284

.30 7.62mm                                     .308

.32 8mm                                         .323

.33 8.3mm                                      .338

.35 8.9mm                                       .358

.36 9.3mm                                       .366

.37 9.4mm                                      .375

.41 10.4mm                                    .416

.45 11.4mm                                     .458

.50 12.7mm                                      .510

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Ron Spomer has been photographing and writing about the outdoors for nearly four decades. He’s written seven books, hunted on six continents and been published in more than 120 magazines. He’s currently rifles’ editor at “Sporting Classics,” Travel columnist at “Sports Afield,” Field Editor at “American Hunter” and “Guns & Ammo” — Optics Columnist at “North American Hunter,” Contributing Editor at “Successful Hunter,” Senior Writer at “Gun Hunter,” and TV host of “Winchester World of Whitetail.” He will write on Shooting Tips weekly for sportsmansguide.com. You can read his blogs and catch some of his YouTube videos at www.Ronspomeroutdoors.com.



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