Andrew’s Note: Prepography reader CCP wrote in to ask: “Why is the .223 caliber designated as 5.56mm by the military? Also, why is the .308 caliber designated as 7.62mm? 25.4mm = 1 inch. I thought it was a great question so I put it to the guy who taught me the difference between .223 caliber & 5.56mm…my buddy, Infidel…
Thanks for your question, I was going to do a article on the difference between the .223 and 5.56 chamberings anyway so I’ll discuss that as well.
.223 and 5.56 are different beasts. In a nutshell, a direct quote (and worded better than I could have) from Wikipedia, “The 5.56mm NATO chambering, known as a NATO or mil-spec chamber, has a longer leade (also referred to as the throat), which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet.” This longer leade allows the 5.56 to run higher pressures you cannot in a .223 chamber. This is the very reason you cannot shoot 5.56 out of .223 chambers. If you do you can cause damage to your firearm. At the very least you will have popped primers and extraction problems. Worst case you will blow up an AR (short for Armalite Rifle) really quick.
If you have purchased an off the shelf AR, Bushmaster, Rock River, DPMS etc, your chamber quality is a crap shoot. Even though most of them are marked 5.56, they will not be true 5.56 chambers. The profit margin is not very much on AR’s. So manufacturer’s when making barrels, will not replace the chamber reamers until they get very dull. Thus if you happen to get a barrel at the end of the life of a chamber reamer, you will not have a true 5.56 chamber. And for some reason manufacturers will not make MIL SPEC chambers anyway. There is only one company that makes anything true MIL SPEC and that is COLT. However, some of your high end guns, Bravo Company, Novenske, LMT, etc. do make good chambers.
Now if you bought a off the shelf (what I call) lower end rifle like a Bushmaster, Rock River, DPMS etc., and it has the occasional fail to eject, won’t feed steel cases, or short strokes (fails to fully cycle the action and load another round) there are some things your gunsmith can do.
Steel cased ammunition is another story. Now I will say I don’t digest a lot of steel cases in my ARs because I don’t think that steel on steel is good for long term use…that said, my guns will shoot it reliably. Now I know I will cause some arguments here with conventional AR wisdom… but I am that way! There’s a theory that the lacquer used on steel cases causes guns to stop running. I don’t buy that theory, because why would it not do the same thing in an AK or 7.62×51 which regularly use lacquered steel cases? I know some will argue with me, that is fine. I just don’t buy it as my guns run fine with steel cases…but remember that I’m shooting a true 5.56 chamber.
On .308 vs 7.62×51, from what I have always heard and researched is you can shoot either caliber in either chambering. If you research it, you will get conflicting information though. I just don’t know enough about this combination to have a strong opinion even though I have guns with both chamberings. Factory ammunition and my reloads all seem to work fine. I don’t reload to max pressures of commercial .308 so I have never had problems. [Andrew's note: .308 commercial is a higher pressure cartridge than 7.62 NATO see this article for additional information.]
So, the bottom line is 5.56×45 and 7.62×51 are NATO standard designations used by military forces and differ slightly from the commercial calibers of .223 and .308. Except as mentioned above (shooting .223 caliber in a 5.56mm chamber but not the other way around) you should only use the ammunition specifically designed for your firearm. I don’t know who made the actual decisions that resulted in this confusing situation but that’s how it is. Most of us will not use NATO spec ammo but if you do and have problems, it can be fixed as described above in .223 calibers. .308 probably has fixes, just never looked into it or studied it much. Thanks again for your question.