This “warning” has been around, and around, for years, but it’s still not always heeded, or understood. Read why and how it matters HERE.
Glen Zediker
I know this is “Reloaders Corner,” but, every now and again at least, I rip open the end of a cardboard factory cartridge box, or five.
I just got finished building up a “retro” AR15 for a new book. Reasons for that are a few, but probably the main one was that I wanted to recollect the one that “got away,” well, the one that I let go. Errant short-sighted judgment, as is common in youthful people. So I built a replica M16A1, circa mid-60s, well, of course, with only two selector stops. At the heart of that rifle is an original-spec barrel, chrome-lined, NATO chamber.
That’s leading to this: I opened up a few boxes of “genuine” NATO 5.56 to check it out with, something I honestly haven’t fired for years and years. Dang. That stuff is potent. Over the past several years, the pressure level has increased. Current standard is a little over 62,000 PSI. (NATO is technically measured differently than commercial, but the figures I give here are accurate for comparison.) Compared to SAAMI specs for .223 Remington (commercial) that’s a solid 7,000 difference. (That SAAMI-spec figure has likewise increased over the years, judging from recent test figures I’ve seen respecting commercial .223 Rem.; most references heretofore were max at 52,000 PSI.)
The main impetus for this article, though, came from a recent experience at a local gun shop. I went in search of a sub-sonic .300 Blackout load, and they had one in .300 Whisper. The counter person told me that it was “exactly the same as .300 Blackout, just like .223 is the same as 5.56…” Whoa. Neither statement is true, although Whisper specs are plenty close enough to Blackout that no differences factor in safety or function. However! I didn’t take the time to lecture, but, dang, .223 Rem. and 5.56 NATO are not nearly the same.
First point: do not fire NATO-spec ammo in a rifle with a chamber marked “.223 Remington.” It will, not may, be over-pressure. Reasons have to do with chamber specifications for 5.56x45mm NATO and those for SAAMI-spec .223 Remington. There is a significant difference in the leade or “freebore” cut comparing SAAMI to NATO. That’s the space in a chamber ahead of the cartridge case neck area that leads into the rifling. NATO is radically more generous, meaning “bigger”: longer, more volume. (About 0.150 inches, based on my measurements of bullet seating depths that touch the lands.) There is relatively much more room for expanding gases to occupy in a NATO chamber. In a SAAMI chamber there’s much less room for expanding gases to occupy. The additional pressure is about the equivalent of another full grain (or more) of propellant in the case. Yikes.
There are other little nit differences to pick between the SAAMI and NATO cartridge, and, therefore, chambering specs, but they don’t really factor in a material sense. There’s bound also to be just as many small differences in cartridge dimensions from one maker to the next. I’ve measured enough to tell you that’s true.
Now. What this has to do with reloading (finally, I know) is based on a question I’ve gotten over the years, a concern to some, or at least, as said, a question. And the answer is that you’re better off going with .223 Remington loading data for any ammo intended for “general” range use. That means blasting away on an afternoon. Just because it’s a NATO chamber does in no way mean you’re supposed to run NATO-spec ammo through it! Back it off and enjoy it more.
If you’re relying on a factory-published data manual to give a place to start, or stop (something from Sierra, Hornady, Lyman, or so on) pay very close attention to the test barrel specifications. Clearly, barrel length has a big influence on attaining the published velocities, and some load combinations are going to be worked up using considerably longer barrels than what the most of us have on our AR15s. But the biggest factor is the chamber used in the test barrel. If it’s a SAAMI-spec (sometimes called a “SAAMI-minimum”) chamber then the data should be on the conservative side. Should be. Do not, however, bank on any idea that you should jump straight to the maximum load listed if you’re loading for use in a NATO. There are, always, too many factors that otherwise create more or less pressure (primers, cases, propellant lot, and more).
As time goes by it probably is less likely to encounter a semi-automatic “.223” that’s not a NATO, but it will be marked as such! Clearly, most ammo is used in the most popular guns. That’s not going to be a bolt-action anymore. Make no mistake, though, AR15s exist plentifully that have SAAMI chambers, and I see a lot of aftermarket barrels that are cut with that minimum-dimension reamer.
ANOTHER OPTION
So what’s a “Wylde” chamber? This is a chambering spec developed by Bill Wylde, one of the early and leading pioneers in the quest for improved AR15 accuracy. It is popular and available, especially in aftermarket barrels. What it is, is a chamber that’s in-between SAAMI-minimum and NATO, leaning closer to NATO. Rumors are true: it’s safe to fire NATO-spec factory loads through a Wylde. The Wylde was designed upon the introduction of the heavier competition bullets with the idea of providing more freebore to accommodate the necessarily longer cartridge overall lengths necessary with something like an 80gr. Sierra, but keep the amount of jump to a minimum with shorter bullets fed from the magazine.
This article is adapted from Glen’s books, Handloading For Competition and Top-Grade Ammo, available at Midsouth HERE. For more information about other books by Glen, visit ZedikerPublishing.com
