By Glen Zediker:
I hope by now that most folks do know there is a difference between 5.56 NATO and .223 Remington. Either way, I’ll be hitting that topic eventually, and the reason is because it really matters! NATO is loaded to much higher pressure than commercial .223 Rem. There’s up to 15,000 PSI difference. Reason: the NATO has a radically longer leade or throat, more room for expanding propellant gases.
But, what about 7.62 NATO compared to its civilian equivalent, .308 Winchester? Good question.
A little front-loading: There are industry standards for cartridge dimensions and operating pressures. SAAMI — Sporting Arms and Ammunition Manufacturer’s Institute — is a voluntarily-given base of information that defines and delineates our modern centerfire cartridges. Essentially, it’s an agreement among those in our industry to produce cases and cartridges and chambers that fit the specs, and they all agree to the specs. That’s for the U.S.A. SAAMI defines minimum and maximum headspace, for instance, and also chamber pressure maximums.
NATO—North Atlantic Treaty Organization—is the outfit that standardized mil-spec ammunition in use by the allies, including U.S. Armed Forces. It has its own set of standards for dimensions and outputs.
So. Comparing .308 Winchester and 7.62X51: .308 Win. came first. The two ended up differently… Like the 5.56/.223, the difference between commercial SAAMI-spec .308 Winchester and 7.62 NATO is in the chamber, but it’s in a different place. The difference in these two isn’t the throat, it’s the headspace. NATO is ridiculously longer. Although there are different pressure-testing standards used by NATO and SAAMI, .308 commercial is normally loaded to a higher pressure maximum than mil-spec NATO. It’s safe to shoot NATO rounds in a .308 Win. chamber. The difference comes in shooting a commercial .308 in a NATO chamber. At the least, the commercial case will be stressed, a whopping lot. Numbers?
Sho. SAAMI-spec headspace for a .308 Win. is a minimum (shortest allowable headspace or “GO” figure) of 1.630 inches, and a “reject” (too long) figure of 1.634. Well, the number given as a minimum for NATO is 1.638. The “reject” on NATO is 1.6455. I’m not at all clear on why anyone thought that was a good idea, but that’s what was done. It is also the reason that NATO cartridge case specs call for considerably thicker-walled brass. It has to endure that much more expansion. It’s a big reason not to purchase “surplus” 7.62 brass for reloading. Well that’s my advice… Cases fired through a true NATO chamber will be patently worthless to anyone owning a .308 Win. They’ll be blown beyond all reasonable repair.
A “true” NATO round will have a cross inside a circle stamped into the head. You can’t tell by chambering a round, no difference there. The other influential cartridge (not chamber) dimensions (overall round length, headspace, etc.) are patently the same for either standard. Crazy.
The preceding is a specially-adapted excerpt from the new book, “Top Grade Ammo,” by Glen Zediker. It’s soon to be available at BuyZedikerBooks.com or call 662-473-6107. You can also find Glen’s other works at Midsouth Shooters Supply’s.