Making Sense of Cartridge Names

Centuries ago, no soldier lighting the wick on a matchlock could have imagined a self-contained metallic cartridge launching a tiny bullet at three times the speed of sound. Between then and now, lots of arbitrary decisions guided development of ammunition. Probably history wouldn't have been much altered if instead of packing his .45 Colt, Pat Garrett had gone after Billy the Kid with a .44 S&W American, or if in the design of either someone had insisted on a 43-caliber bullet or a thicker case rim. John Plute's huge Colorado elk, whose antlers topped the charts for most of a century, could as easily have fallen to a .30-30 as to a .30-40 Krag. Or, to some 29-caliber round someone once dreamed but never birthed.

Caliber. Everyone talks about it, but the word is commonly misused. The caliber of a rifle or pistol is the diameter of its bore. You can fret about whether that actually means bore (land) or groove diameter; conventionally it is bore. As in 30 caliber. Or 25 caliber. Groove (bullet) diameters for these two are .308 and .257, respectively. By the way, you don't have to put the period in front of the number when writing "caliber." When you fire a high-octane .22, say, a .220 Swift, you're shooting a 22-caliber centerfire. While caliber can be indicated in millimeters, it's awkward to say "7mm caliber." A simple "7mm" is sufficient. The word has nothing to do with chambering. So if you say "my rifle is a .257 Weatherby caliber," you're saying too much. It has a 25-caliber bore--but so do rifles in .250 Savage, .25-06 and .25 WSSM. Better to say "my rifle is chambered in .257 Weatherby."

Exceptions add color to any set of rules, and in cartridge nomenclature, exceptions abound. They have increased as new entries have shouldered into caliber groups already crowded. Differences in stated diameters can reflect differences in bore and groove measure, because cartridges have been named for both. Sometimes numbers shy from either dimension to distinguish a round from others with the same bullet. The .243 Winchester bullet mikes .243 in diameter, same as the rifle's groove diameter. The .244 Remington, whose case differs, uses the same bullet. However, Remington renamed the .244 the 6mm a few years after its 1955 debut. Yes, a 6mm bullet is .243 in diameter. So are bullets for the .240 Weatherby Magnum. A .280 Remington fires a .284 bullet, as does the .284 Winchester and all popular 7mm rounds, from the 7mm BR to the 7mm STW. The 7x57 is a European round. That designation sensibly indicates both bullet diameter and case length in millimeters. The 7x64 carries the same bullets but in a longer case. An 8x57 uses a .323 bullet--a diameter that never caught on Stateside.

Diameter may appear as a two- or three-digit English number (or one-, two- or three-digit metric display). Cartridge designation may also show something besides diameter. The .30-06, for example, appeared first as the .30-03, developed with and for the government's 1903 Springfield rifle. Shortly, a case trimming of .07-inch (to 2.494 inches) made the .30-03 obsolete. The government recalled all Springfields and rechambered them to what we know as the .30-06. The .25-06 and .35 Whelen were its early progeny, developed by wildcatters who simply sized the neck down and up to get higher velocities with light bullets (the .25) and greater bullet weight for heavy game (the .35). Townsend Whelen also put his name on a 40-caliber round from .30-06 brass: the .400 Whelen.

More recent military cartridges have metric names. The .308 Winchester designed for same .308-diameter bullets as the .30-06 and .300 Holland & Holland magnum, appeared in 1952 as a sporting round. It derived from efforts by Winchester and the U.S. Army to replace the .30-06 with a more compact service cartridge that would also work better in machine guns. (Thirty years earlier, Savage had similar hopes for its .300.) In experimental stages the .308 Winchester was the T-65. In 1955 it began a military career as the 7.62x51mm NATO. It would yield to the 5.56x45mm NATO in 1964, four years after the Army chambered this round (to appear in sporting circles as the .223 Remington) in the AR-15. Still popular among target shooters and as a mid-range sniping round, the .308 is commonly known as "seven-six-two" by shooters--many of whom dismiss the AK-47's smaller, less powerful cartridge--the 7.62x39.