It’s vital to understand “which” chamber is in your AR15. What you don’t know can create big problems. Here’s why.
Glen Zediker
I’ve talked over or at least touched upon this topic, here and there, in other articles. And this week I got four phone calls asking for advice on “which” AR15 chamber I’d recommend. I guess that sort of spurred creation of this article. My primary goal (always) is to answer questions, and ideally before they are asked. So…
There are a few options today, and, no, it never was “simple.” There have always been two distinct chambers cut for .223 Remington and 5.56x45mm NATO. And that’s the difference right there! See, .223 Rem. is a commercial round, 5.56 is a mil-spec round. Yes. They are “the same,” but they’re not. The difference is in how these two are loaded with respect to pressures. NATO is a whopping lot hotter. To the tune of +15,000 PSI.
The differences in the chambers are, pretty much, that a NATO has a significantly longer throat or leade or freebore, whichever term is preferred. This is the area in a chamber that extends beyond the case neck cut.
This area in a chamber accepts the initial gas expansion, so, in one way, it can be looked at like an expansion chamber. More room for expanding gases effectively reduces stress on the case. When this area is lengthened, there’s more room, less pressure build. When this area is shortened, there’s less room, more pressure build.
As said, .223 Rem. is short, NATO is long. Take a NATO-spec round and fire it in a .223 Rem. chamber and there’s too much pressure. The .223 Rem. will “fit” just fine; there’s no influential differences otherwise in chambering specifications between .223 Rem. and 5.56.
You’ve probably heard all that before. It’s very important to know. “Which” chamber affects making loaded ammo choices, and also in interpreting reloading data.
Short history as to the reasons these two chambers exist: .223 Rem. in civilian, commercial application was a varminting-type round, along the lines of .222 Rem. When SAAMI (Sporting Ammunition and Arms Manufacurers Institute) laid down the specifications for that round it did so based around the prevalent short .224 bullets of the day, which were often 52-grain flatbase designs. For best accuracy with the little bullets, the throat was kept short, decreasing the distance the bullet had to travel to engage the lands or rifling. Some, most, me included, call this chamber a “SAMMI-minimum.” The mil-spec ammo assembled for M16s used a 55-grain boat-tail loaded to a higher velocity, and the longer throat was specified to handle the extra gas.
What matters is knowing that you don’t have a .223 Rem. chamber. A NATO can handle anything.
Most AR15s I’ve handled in the past good long while have NATO chambers. It’s the only thing that makes any sense for someone, anyone, who wants to fire sto-bot ammo. Not all the mil-type commercial loads (like the “white box” varieties) are true NATO spec, but if the ammo is not marked “.223 Rem.” it might be a tad amount to a lot hotter than a short-throated gun should handle. True NATO ammo has a distinct marking on the case base.
There is now another what’s become “standard” chamber for AR15s, and that’s the Wylde. Named for AR15 accuracy pioneer Bill Wylde, this reamer specs fall between SAAMI-minimum and NATO. Bill started cutting these chambers for NRA High Power Rifle contestants who needed more room in the throat to accept the long 80-grain bullets but not so much room that the shorter 69-grain bullets were having to leap a gorge to engage the lands. A compromise. A Wylde is a good chamber, and a good choice.
Here’s how it breaks down, according to me:
SAAMI-minimum or commercial .223 Rem. chamber is good for those who are wanting the best accuracy from light bullets. Can’t run mil-surplus ammo or NATO-spec commercial though.
NATO is for anyone who wants to shoot anything and everything out there safely.
Wylde is more or less an “Improved NATO,” and my experience has been that it will safely handle true NATO loads, even if that’s not its intended design. I base that on spent case condition. It will shoot a little better than a NATO with lighter, shorter bullets. The Wylde is available more and more commonly now from different manufacturers and in “drop-in” accessory barrels.
The preceding was adapted from Glen’s newest book, Top-Grade Ammo, available here at Midsouth. For more information on this book, and others, plus articles and information for download, visit ZedikerPublishing.com
