The confusing world of rifle caliber

Debra and I went up north again over Thanksgiving, spending four days and nights at a ranch in Ranger, TX with our friends Mike and Kristi. Mike spent part of each day sitting in a deer blind with his rifle, hoping to get some fresh venison. One afternoon I was asking him about his rifle, which he said is a “thirty-aught-six,” and we got to discussing the different types and calibers of rifles.

Understand, I’m not completely unfamiliar with firearms. I consider myself more knowledgeable than most in that I qualified Expert with handgun and rifle, can break down, clean, and re-assemble a weapon, and have a working knowledge of guns in general and a healthy regard for gun safety. But I’m certainly no expert or walking encyclopedia on the topic. In particular, I find the whole issue of caliber to be incredibly confusing.  I expect things to make sense, and if you look only at the caliber, things make no sense at all.

We’ll start with Mike’s “thirty-aught-six,” more commonly written as “.30-06.” The “.30” part means that it’s a .30 caliber bullet. That is, the bullet is .30 inches in diameter. The “-06” in the designation stands for the year 1906, which was the year that the United States Army started using it. The designation covers more than the bullet. The Springfield .30-06 is 7.62 mm (diameter) x 63 mm (length) cartridge that has a specific shape and load (amount of powder). This became the standard round for the U.S. Army and was used for almost 50 years, including in the WWI era Springfield rifle and the WWII era M1.

So the .30-06 is a .30 caliber rifle. How does that differ, for example, from a .30-30? A .30-30, as you would expect, is a .30 caliber round, but what does the “-30” stand for? It’s not the year in which the round was introduced, but rather the size of the load: 30 grains (about 1.9 grams) of smokeless powder. Winchester introduced the cartridge in 1895. It’s a 7.62 mm x 51 mm cartridge, as opposed to the -06’s 7.62 x 63.

And then there’s the .308 Winchester, which is also 7.62 x 51, and is very similar to the 7.62 x 51 NATO cartridge introduced in the 1950s and used in the M14 rifle and M60 machine gun, among others.

Remember I said before that a .30 caliber bullet is .30 inches in diameter? That’s not entirely true. The .30-06, for example, has a bullet diameter of 0.308 inches, as do the .30-30 and the .308 Winchester. The 7.62 x 51 NATO, though, has a bullet diameter of 0.300 inches. The .303 British is 0.311 inches, as is the .303 Savage. With the exception of the NATO cartridge, which really does have a bullet diameter of 7.62 mm, the rest of them are confusing as heck. Why a .308 would be 0.308 inches but a .303 would be 0.311 inches is beyond me. I suspect that the measurement is actually the diameter of the rifle barrel and depends on how the measurement is taken (i.e. between the grooves, between the lands, or maybe from one groove to the opposing land).

It doesn’t get much better with handguns, by the way. For example, I always wondered why a .357 Magnum was more powerful than a .38. I’m not talking a little bit, either. A .38 is a dangerous weapon, no doubt, but it pales in comparison with the stopping power of a .357. Why is that? After all, isn’t the .38 bigger?

Actually, the .38 Special has a bullet diameter of 0.357 inches. Why is it called a .38? I don’t know, but the neck diameter of the cartriged is 0.379 inches. The .357 Magnum bullet is also 0.357 inches in diameter. The .357 Magnum has a heavier bullet and a bigger load (i.e. more powder), making it a much more powerful weapon.

A 9 mm handgun, by the way, has a bullet diameter of 0.356 inches. So a 9 mm is pretty much the same thing as a .38, and there are 9 mm Magnum cartridges that are comparable to the .357 Magnum cartridges.

Face it, the world of guns and ammunition is seriously confusing. And for the most part the caliber doesn’t particularly matter. For example, the M16 rifle uses a 5.56 mm round (.224 inches)–the same size as the .22 long rifle bullet fired by popular .22 caliber rifles and pistols that are used for small game hunting and target practice. The difference is that the 5.56 bullet is larger, heavier, and has a whole lot more power behind it.

A fatter bullet isn’t necessarily “better” when it comes to stopping power. What matters is the amount of kinetic energy, which is one-half the mass multiplied by the square of the velocity. So if you double the bullet’s speed, it has four times the kinetic energy. Faster is better than fatter.

I don’t know that my newfound knowledge is particularly useful. There’s still a lot I don’t know, and I’m okay with that. I’m just happy that I won’t be totally bewildered the next time I get into a discussion of rifles and calibers.

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