Reloaders Corner: AR15 Chamber Options

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…

NATO mark
A TRUE NATO load always has this mark on its base: the cross-in-a-circle stamp. Some commercial ammo that appears to be mil-spec may or may not be, but err on the safe side.

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.

Chamber-All gage
I use a Hornady LNL OAL gage to find out exactly the length of the chamber throat. Get one at Midsouth. This read shows “NATO” by the way. Sierra 80gr MatchKing at 2.550 inches to touch the lands. Wylde should read 2.475. SAAMI-minimum will (usually) be 2.395.

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.

NATO pressure
Here’s “real” NATO fired in a commercial .223 Rem. chamber. Ouch. The imprints and general beating the case head shows are the result of the additional pressure in the NATO loading, and the .223 Rem. chamber’s inability to excuse that much extra pressure.

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.

Compare .223 chambers
Here’s the best way to see what’s going on with AR15 chambers. These are Sierra 80-grain MatchKing bullets loaded to an overall cartridge length that has the bullet touching the rifling. Left to right: SAMMI-minimum .223 Rem.; Wylde; NATO. Wahoo. Big, big differences. There’s a little more than 0.150 inches between the SAAMI-minimum and the NATO and that space in the throat handles the extra PSI of NATO-spec loadings. It is also, by the way, how to know (or one way to know) the actual “length” of a chamber throat.

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.

NATO stamp
There’s a few ways I’ve seen “NATO” marked on barrels, and I’ve seen a good number of barrels that aren’t marked at all. That’s terribly irresponsible. Look for “5.56” since that seems to have become the more common way to denote “NATO.”

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.

winchester .223 ammo
If you have a “.223 Rem.” stamp on your barrel don’t feed it any ammo that is not clearly likewise marked “.223 Rem.” Should say the same on the case headstamp. If it doesn’t read “.223 Rem.” do not fire it in a barrel stamped “.223 Rem.” This ammo is safe for any AR15. If you don’t see a stamp on your barrel, find out…or just fire .223 Rem.

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

9 thoughts on “Reloaders Corner: AR15 Chamber Options”

  1. if my barrel has 556 nato on it should i use 556 nato data when reloading or is it ok to use 223 rem data?.

    1. You can use either, because the 223 is the lower pressure. What they are saying is, is that you can load it up to the nato data if you want to because your barrel is so marked. If you have any questions as to exactly how it was chambered because of the stamping, then call the manufacturer and ask them what their chamber specs are. Bottom line, higher pressure rating, all loads are ok, lower pressure rating, only 223 loads.

  2. This is very good information that could save equipment damage and limb or life! Thanks for that!
    What I would like even more is the measurement to use that tells us / me the distance to the rifling of each round.
    I use a method of measuring a bullet with a comparator and calipers in the case slightly loose so it can move in or out but not easily. I chamber the round (no powder or primer) remove it and measure it several time to ensure that I get the same results each time. You must start with the round protruding the case enough so it will be forced into the case when chambered. Once you have that measurement you can then load rounds a specific distance off the riffling. In my 30:06 handgun that is very close (most bullets are less than .010 off the riffling ) that may not be safe for the AR but if we knew that distance for each we could then determine what chamber we have. Thanks for all the good information.

  3. So who has the liability? Who gets sued? The firearm manufacturer or the ammo manufacturer or both?

    I am sure they know this, which implies they are wiling to accept the risks and liability. (I doubt it)

    You don’t provide the technical info as to what those chamber cuts actually are. Do you really know?

    “As said, .223 Rem. is short, NATO is long. ”

    What is short? What is long?

    “Take a NATO-spec round and fire it in a .223 Rem. chamber and there’s too much pressure.”

    What is too much?

    My point. If this is truly the case, then by now, surely someone has had a gun “blow up” using the “wrong” ammunition. How are those lawsuits progressing?


  4. The big difference is inside the case! NATO ammo has a thicker case mostly near the base (webbing), to withstand the pressures created. Like compression in engines, smaller area, greater pressure. If you load NATO specs in a .223 case you WILL have problems with case separation, blow outs, etc. It may not happen on the first shot but it will happen. It’s simple, if your firearm is stamped 5.56 you can shoot either, it is built to handle the higher pressures, if it’s stamped .223 then that is what you shoot. Knowing about chamber pressures is what will save you and ALWAYS start at recommended powder drop as stated in your manual. Use the right cases, 5.56 cases can be loaded to NATO spec or .223! .223 cases should only be loaded to .223 specs! I had a friend that used to like to load .223 cases beyond specs, and used to laugh every time he shot a round that vaporized around twenty feet from the barrel! Waste of money, wears out cases, wears out the rifle sooner, and NOT SMART! What gets me is, people will buy something and NEVER read the manual that comes with the toy purchased! Then when all hell breaks lose they are immediately suing someone for their own negligence. I see these pictures/ stories all the time, and it’s usually the owners fault, with a lot of details left out to cloud the evidence. Best thing is, READ the manual that comes with the new “toy”, and learn reloading, if you are going that route, from an experienced, knowledgeable, individual, and lock the doors and avoid interruptions, don’t load “under the influence” when doing so. Be safe and have fun!

  5. I have a .223 Wylde chamber. Even if you measure your chamber you are still restricted on length in the magazine

Leave a Reply