Understanding Metric and Inch Bore Dimensions

I often have blog readers and consulting clients send me questions about firearms calibers. They are often confused, but this not always their fault. It is a confusing, complex, and often arcane topic. A lot of the facts that you will find are mere trivia, but since safety is an issue–(we mustn’t fire the incorrect ammo in our guns!)–it is important to study.

To begin: Metric caliber designations are usually written with an “x” in the middle to distinguish the bore diameter and case length. For example, the 6.5×55 designation tells us that the cartridge uses a 6.5 mm bullet, and a case that is 55 mm in length.

The following table is partly Creative Commons licensed (courtesy of Wikipedia), with additions by JWR (such as 7.5mm, 7.7mm, .375 H&H, .455 and .577) and minor corrections and comments. Hence, I am retaining my moral rights.

As you can see, there are some amazing inconsistencies in cartridge designations. (See, for example, .405 Winchester and .38-40.) As Jim Keenan at the Firing Line Forums aptly put it: “It is usually best to just accept cartridge names; trying to figure out reasons for the names leads to insanity.”)

One regular source of confusion in cartridge naming is whether “bore diameter” refers to the rifling land dimension versus rifling groove dimension. For example, most .303 British rifles have a land-to-land diameter of .303 and a groove-to-groove diameter of .311. So when you handload a “.303” cartridge, you actually use a .311 diameter bullet.

Specifications have also changed for a few types of ammunition. One example is the 8mm Mauser cartridge. Early production 8×57 rifles used 8.08 mm (.318 caliber) bullets, but the later guns used 8.2 mm (.323 caliber) bullets.

Now, on to the realm of shotguns, where life is simpler but there is still some minor confusion.

Common Production Shotgun Bores and Gauges in North America

Note that there are presently enormous revolvers being marketed that can fire both .45 Colt cartridges and .410 shotshells. (The Taurus “Judge” series.) I attribute the popularity of these revolvers to: A.) Ignorance of ballistics, and B). The unerring willingness of the American people to spend their money on impractical toys.