Tag Archives: .223 Rem.

Reloaders Corner: AR15 Chamber Options

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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 ZedikerPublishing.com

Reloaders Corner: Pressure Curves and Port Pressure – Part 2

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The following is a specially-adapted excerpt from the forthcoming book,” Top-Grade Ammo,” by author Glen Zediker, owner of Zediker Publishing. Click here to order.

Last time I gave a caution about respecting one of the differences between semi-auto and bolt-action rifles, and that was with respect to propellant burn rates. The summary reason for that is that different rate propellants will “peak” at different areas as the expanding gases and the bullet travel through the bore. Slower-burning propellants peak farther, and that means more pressure is available at the gas port location in an AR-15, for instance, as the bullet passes it. If the system is oversupplied, then the system is overworked.

Compared to ideal function when gas supply is delivered as engineered, mistimed peak pressures can result in the bolt unlocking too quickly and excessive bolt carrier velocity rearward. The system just gets hit too hard. The extractor tries to yank the case out of the chamber too soon, before the case is released from its grip on the chamber walls (from being expanded through firing). Spent-case condition shows a measurably more abused hull. Probably the worst popular example of these effects is the M1A. I’m doing an entire column or two on reloading for this beast. Essentially, a spent case from an M1A will show dimensions that don’t seem possible. These come from the bolt unlocking too quickly. AR-15s actually handle excessive pressure better than some other designs.

Always keep in mind that this is all happening in about 2 milliseconds. Average time a bullet spends in the barrel, for most modern centerfire rounds, is 0.002 seconds. Timing is everything.

Keeping in mind the behavior of a pressure curve, which is like a wave cresting, factors that influence the amount of gas-port pressure, using the same load, include barrel length, gas-port size, and gas-port location. When the bullet is sealing the bore, the longer the barrel, the more pressure is contained for a longer time. The smaller or larger the gas port size, the slower or faster the gas enters the system. The farther back or forward the port is located, the sooner or later. Bullet weight is a factor also: heavier bullets accelerate more slowly (and also the reason heavy bullets erode the chamber throat more than lighter bullets).

And, the amount of volume inside the bore has a huge influence on all this. That matters when we’re using another caliber than .224 in an AR-15 or .308 in a big-chassis AR (like an SR-25). For instance, in that rifle chambered for .243 Win., but retaining the gas system specifications (gas port size and location) of the .308 Win.–chambered rifle, there’s way more pressure only because there’s less space, less volume, in the bore. The opposite is usually true when we’re running an AR-15 with a larger caliber bullet.

Selecting a propellant with a suitable burning rate, which, again, is something in the vicinity of H4895, is really the only thing we can do on the loading bench to ensure that we’re not contributing to these symptoms. Beyond that, dealing with excessive pressure gets technical.

All my NRA Match Rifles, which usually have 26-inch barrels, get their gas ports moved forward one to two inches. These, of course, are custom-barreled. I also usually install an adjustable gas manifold.

Moving the port forward effectively delays the wave of gas moving through the bore, kind of repositioning its peak with respect to its outlet; there is more space available for expanding gases. It also allows a little slower-burning propellant, which can take more advantage of the longer barrel. It’s common in a similarly constructed AR-10 to get a port moved as much as 5 inches forward to accommodate a .243 Win. or .260 Rem. chambering.

The adjustable manifold allows some tuning. There are essentially two forms these take. One way is to restrict or limit the through-flow; the other just bleeds it off. I like the first kind the best.

Also, I have searched far and wide for a consensus on gas-port sizes, and came up empty.

All this changes with different chamberings and rifle configurations. Carbine-length barrels are particularly sensitive to port pressure because the port is located farther back.

There are a few surefire things that will alert you when your rifle is exhibiting “over-function” symptoms, such as spent-case condition showing excessively blown (extended) case shoulders, extractor marks on the case rim, and a generally explosive sensation in functioning.

In a more extreme circumstance, an over-accelerated carrier can “bounce” back from its rearmost travel so quickly that a round can’t present itself in time to be picked up by the bolt, or the bolt stop can’t engage quickly enough to hold the bolt carrier.

Sometimes what appears to be a “light” load is actually not. I’ve seen excess pressure leave a spent case in the chamber because the extractor lost its grip, and I’ve seen chunks pulled right off case rims. That’s severe. That’s also another cause for the “short-stroke” appearance of over-function: the extractor issue has slowed the carrier.

If you’re having any problems with “over-function,” solutions include retrofitting an adjustable manifold, increasing carrier mass, installing a stouter buffer spring. I do all those things on my rifles. Keep in mind that I am primarily a Service Rifle shooter, and I am trying to push an 80-grain bullet as fast as reasonably possible from a 20-inch barrel that can’t get the modifications mentioned. I know a thing or three about delaying bolt unlocking — I’ll cover more on this topic if you all want to know.

 

Sources:

Sun Devil Manufacturing

663 West 2nd Ave., Suite 16

Mesa, AZ 85210

(480) 833-9876

 

Medesha Firearms Ltd.

10326 E. Adobe Rd.

Mesa, AZ 85207

(480) 986-5876