7 May 19

308 and 7.62×51

As most of us know, 5.56×45 is the “militarized” version of the civilian 223 Rem varmint cartridge, developed by Remington in 1957.

Military-rifle chamber dimensions for the 5.56×45 cartridge are more generous than the civilian 223 (SAAMI-spec), although both rounds will chamber and fire in NATO-spec military rifles.

SAAMI stands for “Sporting Arms and Ammunition Manufacturing Institute.” SAAMI was instituted in 1913 in order to establish and exchange consistent small-arms technical information between American factories producing military arms and ammunition.

Today, SAAMI continues to dictate chamber dimensions and pressure standards for all commercial calibers, and all gun and ammunition manufacturers dutifully comply.

Commercial rifles in 223 Rem caliber, intended for non-serious purposes, mostly come with SAAMI-spec chambers, which are tighter than NATO-spec chambers, as noted above.

As a general rule, when a barrel is stamped “223 Rem,” it has a SAAMI-spec chamber, and as noted above, it is intended only for non-serious purposes. Tight chambers boost accuracy, but have no place on high-capacity, military rifles.

Conversely, when the barrel is stamped “5.56×45,” it has a looser NATO chamber, with adequate leade, or freebore (this is a short section of bore just ahead of the chamber that is not rifled, allowing the bullet to jump off the case before it engages the rifling). The combination of generous chamber dimensions and adequate leade is intended to prevent stuck cases, broken extractors, and pressure-spikes in autoloading rifles, particularly when they get hot.

Of course, loose chambers yield slightly less inherent accuracy than tight chambers. However, for serious purposes, this accuracy compromise is insignificant

Accordingly, I always recommend a genuine “NATO-spec” barrel for all 5.56×45 ARs and other autoloading rifles that are intended for serious purposes, and I specify it on all “Farnam Signature” Rifles.

308 caliber was introduced into the American commercial market by Winchester in 1952.

Two years later, DOD militarized it, designating their version “7.62×51 NATO,” and as a result of heavy persuasion on the part of DOD, it went on to be adopted my most NATO nations and remained the “standard” through the 1960s and 1970s.

With additional heavy persuasion on the part of DOD, the 7.62×51 was superceded within NATO in the 1970s by the 5.56×45, and remains there to this day.

We are told the decision has now been made at DOD to supercede the 5.56×45 with Remington’s 6.80×43 (6.8 SPC, introduced by Remington in 2003). How long this conversion will take, and whether or not our NATO allies will go along, once more, is anyone’s guess!

One important note:

While the military 5.56×45 chamber is looser than the commercial (SAAMI) 223 Remington chamber, as noted above, the exact opposite is the case with the 7.62×51/308 Winchester caliber!

Military chamber dimensions for 7.62×51 NATO are actually tighter than the those for the original “308″ chamber dimensions, and I’m not sure I understand why this is so!

In any event, this anomaly has become an issue for manufacturers of military-style rifles in this caliber, as you might imagine.

When you buy an FAL, RA/XCR/M, SCAR, POF Revolution, et al and intend it for “serious purposes,” you want a “308″ chamber (with adequate leade), not the tighter 7.62×51 NATO chamber, particularly when you plan on running a wide variety of ammunition through it.

For one, I want my serious rifle to chamber and shoot anything! When a manufacturer cautions, “use only this brand of ammunition,” I have no interest in owning that rifle, as I have no idea of what ammunition I may be one day forced to use!

With my FS 7.62×51 rifle, I’ve been thus compelled to specify “308″ barrels and chambers.

Once again. “serious” autoloading rifles have to be able to chamber and shoot any kind/brand of ammunition you can find, without stammering, without breaking extractors, without the extractor pulling-through the case-rim, and without the extractor ripping heads off of cases.

It’s a serious issue, and your life may one day depend on your rifle running reliably, despite “field conditions,” continuous lack of maintenance, and funky ammunition!