Sunday, July 18, 2010

Followup to "Calibration"

In my previous post but one, "Calibration", I referred many of the myths surrounding the adoption by the US Military of the 5.56mm round, and also the fact that, while the 5.56mm NATO and .223 Remington rounds are nominally dimensionally identical, they are, in fact, not the same round, and 5.56mm rounds should not be fired in .223 chambered rifles. (Although the reverse is possible.)

Looks like I may have garbled my transmission, and led some folks to believe that I was saying that the dangers of firing 5.56mm NATO rounds in a .223 Remington chambered rifle are exaggerated. So, just in case:

If you choose to disregard this advice,  on your own head be it.

Now, the Ammo Oracle has a good, short article on the subject of What is the difference between 5.56×45mm and .223 ammo?, which states, in part:

...Dimensionally, 5.56 and .223 ammo are identical, though military 5.56 ammo is typically loaded to higher pressures and velocities than commercial ammo and may, in guns with extremely tight "match" .223 chambers, be unsafe to fire.

The chambers for .223 and 5.56 weapons are not the same either. Though the AR15 design provides an extremely strong action, high pressure signs on the brass and primers, extraction failures and cycling problems may be seen when firing hot 5.56 ammo in .223-chambered rifles. Military M16s and AR15s from Colt, Bushmaster, FN, DPMS, and some others, have the M16-spec chamber and should have no trouble firing hot 5.56 ammunition.

Military M16s have slightly more headspace and have a longer throat area, compared to the SAAMI .223 chamber spec, which was originally designed for bolt-action rifles. Commercial SAAMI-specification .223 chambers have a much shorter throat or leade and less freebore than the military chamber. Shooting 5.56 Mil-Spec ammo in a SAAMI-specification chamber can increase pressure dramatically, up to an additional 15,000 psi or more.

The military chamber is often referred to as a "5.56 NATO" chamber, as that is what is usually stamped on military barrels. Some commercial AR manufacturers use the tighter ".223" (i.e., SAAMI-spec and often labeled ".223" or ".223 Remington") chamber, which provides for increased accuracy but, in self-loading rifles, less cycling reliability, especially with hot-loaded military ammo. A few AR manufacturers use an in-between chamber spec, such as the Wylde chamber. Many mis-mark their barrels too, which further complicates things. You can generally tell what sort of chamber you are dealing with by the markings, if any, on the barrel, but always check with the manufacturer to be sure.

{All links in the above quote go to various Wikipedia articles. Emphasis in second-to-last sentence added}

The Ammo Oracle lists markings on several manufacturers' barrels, and also has a graphic which shows the respective specifications for the 5.56mm and .223 Remington chambers. ("Throats", actually.)

All I wanted to do was post the links to the post on the Firearms Blog about the ammo giveaway at  That'll teach me to take shortcuts--maybe!


Sailorcurt said...

I think the miscommunication was less a function of what you wrote than it was a function of me not reading well enough, or not thinking well enough about what I was reading, or something.

But thanks for clarifying anyway. I'd hate for someone to blow up a perfectly good .223 Remington barrel because they thought 5.56 Nato is the same thing.

D.W. Drang said...

I thought they were the same for years! I'm pretty sure I didn't learn otherwise until I after joined, after the AWB expired. (As I sometimes tell folks, I had little to no interest in the Ar15 until well after I retired from the Army.)