Chamber choices: 1st commandment: know thy dimensions.
First, it matters. Know about the chamber in your AR-15!
The reason it matters is there are huge (huge--there, I said it twice) differences in chambers cut for .223 Remington. As far as I have been able to research, there's more difference than in any other popular chambering. And the reason for this is because the most popular chambering for this round isn't .223 Remington, it's 5.56 NATO (technically 5.56x45mm NATO). Right, that's the mil-spec version of the commercial .223 Remington (which I'll designate as .223 Rem from here on to save ink). The NATO round preceded it, but when the commercial blueprints for the then-new .223 Rem round were drawn up, it was perceived as most useful as a varminting cartridge, mildly more powerful than the .222 Remington. To that end, commercial chamberings incorporated the architectural standards for other similar varminting rounds.
There are significant differences in the chamber throat area comparing NATO and .223 Rem. The NATO has a whopping lot more space ahead of the bullet. This space creates a likewise whopping lot more room for expanding gases to occupy, and thereby effectively dissipate. That means lower firing pressures.
The chamber throat is the area that extends forward from the case neck area in the chamber to the first point of major diameter, which is land diameter. It's also referred to as the "leade."
The reason the .223 Rem was given a shorter throat is because, with shorter bullets, there's less "jump." Jump is the distance a seated bullet must travel to engage the lands or rifling in the barrel. Usually, that's right: the less gap, the better accuracy. The NATO chamber stemmed from a need to allow use of ammo from different manufacturers of mil-spec cartridges. Some ammunition from different makers around the world are loaded differently, despite all being badged as "NATO." NATO ammunition is also universally loaded to higher pressures than .223 Rem.
Ideally, well according to some, the optimal architecture would have the bullet in a chambered round holding just shy of the lands. A common, effective and related tactic handloaders use for better accuracy is to adjust overall cartridge length so the chambered bullet is just touching the lands. The idea is to have zero jump. Bullets have different contours as they transition from major diameter (the body or shank) to the bullet tip. The curved transition is the ogive. Generally, bullets with a more gradual taper are more tolerant of jumping greater distances to engage the lands. Different bullet makers have their takes on what works best, so it's rare to find two bullets exactly the same.
That, of course, is the problem in getting too wrapped up on chambering. Anyone who wants such theoretical perfection in chambering would have to cut the snoot for a specific bullet, then the same bullet seated to provide, of course, the same cartridge overall length (OAL) each round. Those who indulge in this use a tool called a "throating reamer" to control the thousandths-level precision necessary for lands-on seating. One drawback to this approach is throat erosion. The throat gets longer as the barrel wears, and then the magic is gone... Unless, however, enough math was applied to anticipate, say, 0.050-inch worth of erosion and retain a magazine-length (2.255-inches) cartridge OAL at that point. This means cutting the throat so a 2.245 OAL touched the lands on the freshly cut chamber. This is too complex.
Bullet selection matters the most to accuracy when there's jump involved.
Ever since the AR-15 became a viable tool in NRA High Power Rifle competition, alternate chamber reamers have appeared. These were created to accommodate the generation of longer, higher ballistic coefficient bullets (the existence of which was responsible for the AR becoming a legitimate longer-range competition arm).
One of the most common of these newer reamers is the "Wylde." Named for Bill Wylde, a major pioneer in AR-15 performance, this tool cuts a chamber better suited for the longer, heavier bullets common to High Power competition. I like this chamber. It's shorter than NATO for better performance with shorter bullets, and it's also capable of handling the higher pressures of NATO-spec ammunition. I've been seeing commercially-made AR-15's listing the Wylde chamber as an option.
HOW TO CONFIRM
So how do you know about the chamber in your AR-15? First check what's stamped on the barrel, if there's a stamp. I've seen several that weren't stamped. If it reads "5.56" (at minimum) it should be a NATO chamber. The ".223" designation usually means it's a SAAMI commercial ... usually. Again, there is an increasing number of barrels not marked at all, or with some obscure reference. Chrome-lined barrels are usually NATO. Heavy contour commercial barrels are often .223 Rem.
All these "usually" and "should be" references don't provide the sort of confirmation I know I prefer, so here's how to find out. Or at least one way. I measure the throat using a tool actually intended to assist handloaders in determining the OAL cartridge length when a bullet is touching the lands. To do this, you have to have a reference point, and I do because of the many times I've recorded bullet seating information. A NATO headspace gauge can work sometimes, but it doesn't specifically indicate throat length. However, the bolt should not close on a NATO "go" gauge put into a .223 Rem chamber. Headspace dimensions are longer for NATO.
There are smaller, less influential differences in NATO and SAAMI reamed chambers, but the throat really matters. NATO is loaded to higher pressures than SAAMI. This can be to the tune of 15,000 psi difference. That much, folks, can cause ruptures and even upper receiver cracks. Another issue is NATO rounds can sometimes "stick" the bullet into the lands in a SAAMI chamber. That radically raises pressures. I learned this the hard way. Pay attention to loading manual-provided OAL maximums for different bullets. Don't assume that it's safe to load just any 0.224-inch bullet to a cartridge OAL length that fits into an AR-15 magazine box. It's not. Sometimes, due to the profile of the bullet nosecone, it can stick into the lands prior to firing. And then blow primers, rupture cases, and ...
Clearly, if you're shopping for a new barrel or rifle, get one with a NATO chamber. Whatever you might be sacrificing in accuracy (which honestly isn't much, not based on my groups running 52-grain bullets through NATO chambers) is overcome by increasing the amount of available ammo.
The either/or designations on ammunition and on barrels are converse in their effects. If an ammo box says "5.56/.223" that should mean it's loaded to commercial standards. If a barrel is marked "5.56A223" it should mean it's been cut with a NATO reamer. Just remember: .223 Rem is safe in NATO, but NATO is in not safe in .223 Rem.
Last word: Keep the chamber clean! I use a .357 caliber pistol bore brush and solvent each time I clean the barrel.
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.
1400 West Henry Street, Sedalia, M0 65301
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Satern Custom Machining
33 S. 18th St.
Estherville, IA 51334
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|Title Annotation:||UP on AR's|
|Date:||Jan 1, 2015|
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