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Chamber Choices

Chamber Choices
NATO or Remington? You need to know before
you squeeze the trigger.

Well, this isn’t the first time I’ve done an article on this topic, and it’s probably not going to be the last one either. And this time, as always, I hope folks pay close attention to why this keeps surfacing. For me, it’s the calls and notes I receive and the problems I solve fromwith…Yes, I can create my own words. I studied William Faulkner at the University of Mississippi; it worked for him, even though he failed freshman English.

There are a whole lot more AR-15’s with new homes now than last year, and that also means a whole lot more new AR-15 owners. Along with the rifle purchase, there’s a need for something to feed it. There’s extreme variance comparing AR-15 chambers, reflected through, if not due to, the variances in available ammunition. It is very necessary to know the chambering in your AR-15 barrel.

There are significant differences between “.223 Remington” and “5.56mm NATO.” Most folks refer to either as .223—which worsens the likelihood of potential problems. The two rounds look the same and, for the most part, measure the same. The main difference can be the load within. NATO-spec ammunition can be significantly “hotter,” to the tune of 15,000 PSI chamber pressure difference, plus or minus. It’s “plus” if a NATO 5.56mm round is fired in a true “.223 Remington” chamber, sometimes called a “SAAMI” or “SAAMI-Minimum” chamber.

In most other rifle chamberings, whatever cartridge is stamped on the barrel can be purchased and used without much trepidation. Something like a .270 Win or one of the WSSM’s, and most others also, encounters a range of ammunition from different makers that will suit virtually all factory rifles.

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This is a result of firing a NATO-spec round (above) in a .223 Rem (SAAMI-Minimum) chamber. It held together, but the shiny marks are from the case head slamming back into the bolt face. Fortunately, only the case got hurt. If you want to shoot this (5.56x45mm NATO) make sure you have a NATO chamber. Look for the little “cross” stamp (below) on all NATO rounds. Glen has seen these also come from cartridge boxes labeled “.223 Remington.” Scary.

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Big And Small

I don’t know exactly how many chambers there are for AR-15’s, but it’s more than a few. There are custom reamers in use by specialty builders and the most common, the “Wylde” (named for AR-15 accuracy pioneer, Bill Wylde) can be found ready-to-go in some aftermarket barrels. The most common, however, and the focus of this article, are “.223 Remington” and “5.56mm NATO.” These also represent extremes: The .223 Rem is the smallest and the NATO is the biggest. “Small” and “big” refer to the leade area. (There are very small differences in some blueprints that constitute specifications for .223 Rem and 5.56 NATO—but these are in “tolerances,” for case neck dimensions, for instance, and not likely even a little bit influential in use.)

A chamber is cut with a tool called a—hmm—chamber reamer. This cutter essentially replicates a cartridge case, for one, and next, reaches beyond the cartridge case mouth area to form up the segment that introduces the bullet into the rifling. This area, which extends forward, is called a “leade” and sometimes called a “throat.” The leade dimensions are substantially significant.

Bullets are, well, bullet-shaped. Most .224-caliber barrels have a 0.219-inch land diameter (lands are also known as “rifling”). The “.224” is the groove diameter. Along the surface of every bullet, there will be a point of its diameter that corresponds with land diameter. How far “up” the bullet this point exists—measuring from the bullet base—depends on the contour of the bullet. Some bullets are longer, and, if they are all seated to give a cartridge overall length that will fit into the magazine box, then the distance between this point on the bullet and the corresponding point within the throat, is usually referred to as “jump.” That’s pretty descriptive, because it’s a gap the bullet has to jump to engage the rifling.

In a commercial .223 Rem chamber, this distance is w-a-a-ay shorter than in a NATO chamber.

I imagine this next one is covered somewhere by gas pressure laws (or so my mathematically-inclined son says) but essentially, when there is less space for a gas to expand into, there’s more pressure created in the space there. As I mentioned, NATO ammunition is loaded to hotter specs than commercial .223 Rem. Firing a NATO-spec round in a short chamber can escalate pressures to the point of case failure. That means a rupture and possibly a gas blast back through the charging handle gap, quite near to the proximity of the shooter’s eye.

Making matters (much) worse, not all barrels are marked congruently, which means consistently, and some are not marked at all. So how do you know which chamber you have? It has to be measured.

The easiest and most direct means, by my take, is through the use of a gauge allowing measurement of the leade area available. Otherwise, a chamber cast can show it. I don’t know anyone who doesn’t get paid to do this who wants to do this. Get the gauge. It’s handy for other reasons also, like how it was intended to be used originally—which is to measure the overall cartridge length of competition-use rounds. Competitive shooters might be concerned with the amount of bullet jump in a round, and the less the better, most of the time.

If you’re shopping for an AR-15—either a complete rifle, another upper assembly or replacement barrel—go with NATO. NATO can handle virtually any round built around the 5.56×45/.223 Rem platform. Conversely, if you choose the .223 Rem chambering, then choose ammunition designated as “.223 Rem.” This part here is another mess. I’ve seen ammo for sale with “.223 Remington” printed on the box end flaps, but was, in fact, NATO-spec. With ammo availability as scarce as it is, it only makes sense to choose a chamber to allow the use of anything available.

Folks, this is a serious issue and has created many problems for many people, so take it seriously.

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Here’s an illustration of a chamber leade or “throat.” The influential distance is to the first point inside the bore where the bullet will contact the lands or rifling and, more specifically, how much gap exists ahead of the cartridge case mouth. Different reamers make for different specifications. This distance is radically shorter in .223 Rem compared to NATO.

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These are Sierra 80-grain MatchKings seated to engage the lands for a NATO chamber (left) and a SAAMI-Minimum .223 Remington chamber. Radical. There is over 0.150-inch difference in the space ahead of a chambered round. This is an effectively massive amount more space for expanding gases to occupy, and that’s the influence on pressures inside the chamber.

Shameless Self-Promotion

The preceding is a specially adapted excerpt from The Competitive AR15: Ultimate Technical Guide, a book by Glen Zediker and Zediker Publishing. For more information visit www.zedikerpublishing.com or call (662) 473-6107.
By Glen Zediker

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