Ask a Weapons Man: 5.56 vs. .223?

So here we are with a reader question. About:

The “controversy” over the differences between the 5.56 and .223, both in the ammo and chamber specs…. I thought I was fairly knowledgeable on gun stuff and had never heard that there might be differences significant enough to preclude firing 5.56 ammo in a .223 chambered firearm.

I decided not to routinely fire my 5.56 ammo in my .223 chambered AR. Although the chances for a catastrophic failure are slim (to none) I read too many opinions that some degrading of the chamber might occur over time. The reverse, firing .223 in a 5.56 chamber does not present any problems, other than some loss of accuracy. I notice that many national ammo retailers advertise their AR15 ammo as 5.56/.223, indicating they apparently see the rounds as interchangeable. What say you?

(from the comments to TW3 # 3, edited)

It’s understandable that people get worried about this. There are a lot of internet tales out there. One of the most common is that a 5.56mm NATO round will blow up a .223 chambered rifle. Stop and think about that one for a second. Given that most treat the two as interchangeable, people chamber surplus ammo in .223 chambered guns every day. But if you search the intertubes, you can’t find the kB!s that would result if this were true. Supposedly, this did happen before the Internet Time Horizon to a number of .223-chambered Mini-14s. Proof is lacking.

But the Sporting Arms and Ammunition Manufacturer’s Institute, the guys who own the .223 Remington standard, say that 5.56 is unsafe in the .223. Here’s a cache of their previous warning (they’ve reorganized their website into uselessness).

For most purposes, the two are interchangeable. But there are differences between the 5.56mm standard and the .223 Remington standard, which primarily show up in the chambers of weapons, specifically in the throat (sometimes called “leade”) of the chamber. The .223 chamber is “tighter,” or more abrupt, making it more suited for accuracy; that’s why target weapons tend to be chambered in .223. But the SAAMI spec is also different from the 5.56 specs in both the pressure tolerated — SAAMI is lower — and the way the pressure is measured. This is the source of the kB! fear. But the differences in measurement technique (a tend to work themselves out; the European equivalent of SAAMI, the CIP, sees the two rounds as equivalent, even though thanks to different measurement techniques all three have different pressure limitations. The different agencies measure chamber pressure in different places, and with different piezoelectric sensors.

Pressure limits:

NATO 58,740 PSI

SAAMI 55,000 PSI

CIP 62,366 PSI

(Technically, the NATO and CIP are stated in bar, not PSI, that’s why they’re no round numbers like SAAMI; but the conversion’s just arithmetic. Again, the variation in measurement technique means these three are all just about the same thing).

Military rounds do tend to be hotter both in pressure and in velocity than civilian rounds. But the differences are not large, and are caused primarily by liability-conscious civilian manufacturers downloading the round compared to either of the military specs.

But wait! You said 5.56 “specs”? Well, yeah. There are actually two military 5.56 specs for ammo and chamber dimensions: the US one, which was adopted by NATO, and the FN one. The difference between the two is vanishingly small, which may be why FN’s didn’t catch on. On the canonical FN SS109 round and chamber, there’s actually two angles to the neck — a main one that matches the  46 degrees of the .223 and US 5.56. and a second one forming a sort of easement between the neck itself and the shoulder.

Apart from that unusual detail, seldom seen in the wild, he cartridge cases are the same dimensions, within the tolerances of ammunition manufacture (and the cases of both generations of military ammo, M193 shown here, are the same, too. Yes, the official specs are metric). Those tolerances are remarkably tight for such a small, cheap, and disposable thing as a round of fixed rifle ammunition, because we’ve known for hundreds of years that consistency is the key to dependability and accuracy in ammunition. So the millions and millions of 5.56/ .223 rounds spit out by factories are made as nearly identical as practical, given the machinery and the task.

Now, let’s throw another complication into this witches’ brew. Or two. First, there are other chambers than the military and SAAMI / CIP chamber. Those include the Wylde chamber, used in Rock River’s AR-15s and Armalite’s stainless barrels, and other less common variations. These chambers are generally designed to work reliably with all 5.56 and .223 rounds nd usually feature a throat that’s intermediate between SAAMI and NATO specs. Second, and here’s the big one: the label on the rifle may not identify the actual chamber inside! Some 5.56 chambers are labeled .223, and vice versa. There is a list of these here at The only way to be really sure is to cast the chamber (in plaster or RTV) and then apply a micrometer to the resulting cast — after accounting for the casting medium’s shrinkage, naturally.

So how do you know if your gun is safe? Here are a few rules of thumb:

.223 ammo that is loaded to SAAMI specs is always safe.

5.56mm ammo that is loaded to US or NATO specs is usually safe.

The nature of steel metallurgy is such that, the sort of overpressure the “wrong” ammo could theoretically generate will not fail or deform your barrel, bolt, receiver or other stressed parts. (Now, a double-charged round, or firing with a squib obstructing the barrel, that’s a different thing).

You can check for signs of overpressure (and should) any time you start firing a new gun, start firing a new load in an old gun, and especially when you first shoot .5.56 in a .223. Armalite’s advice here is good:

The first few rounds of ALL ammunition, from whatever source or lot, should be checked for signs of pressure or any other defect before firing large quantities. If you have a problem, you can generally bet that the ammunition meets neither SAAMI nor NATO specifications.

Signs of high pressure include: primers that are blown-out or flowed back into the firing pin hole, flattened primers, powder stains or signs of high-pressure jets (the last only visible under magnification). High pressure can also cause violent cycling in gas guns, high cyclic rates in auto weapons, and cases that are bulged or suffer head or neck separations (these can also be caused by other things, but primer and powder anomalies are almost always signs of unsafe pressures). This doesn’t mean you need to be paranoid, just know “what right looks like” and make sure your fired casings still look that way.

If any of those are found, discontinue the high pressure lot of ammo forthwith. Try it in a looser gun if one id available, or break the amnunition down and recycle the components. No ammo bargain balances against gun damage or personal injury.