CHAMBERS FOR THE .223 REMINGTON AND 5.56mm CLICK HERE to download this document in Microsoft Word format.
The .223 Remington is the most widely-used centerfire rifle cartridge in the developed world. In its 5.56x45 military form, it is the primary issue ammunition for the U.S. Military and NATO forces. It is a popular sporting cartridge, and probably the most commonly used centerfire varmint cartridge. In our Readers' Poll, the .223 Rem (both standard and improved) ranked first among preferred varmint rounds. The .223 Rem is efficient and versatile. It can sling 40-grainers past 3650 fps, and deliver 90gr VLDs accurately at 1000 yards. Its parent case, the .222 Remington, was once a mainstay of benchrest competition. Today, with custom match bullets, the .223 Remington can still deliver impressive accuracy, shooting well under quarter-MOA in a good rifle.
.223 Remington Cartridge History The .223 Rem was derived from the .222 Remington, a round popular with benchrest and varmint shooters in the 1950s. The U.S. Army was looking for a new, high-speed small-caliber round to replace the .308 Winchester (7.62x51). To increase velocity with a 55gr bullet, the U.S. Military bumped the .222 Rem's case capacity by pushing forward the shoulder and shortening the neck. This military modification of the .222 Remington was originally called the .222 Special but was later renamed the .223 Remington. In military metric nomenclature, the round is called the 5.56x45. For the full history of the 5.56x45 cartridge, read the 5.56x45 Timeline, by Daniel Watters.
223 Remington vs. 5.56x45--Chambering and Throat Considerations Is the .223 Remington the same as the 5.56x45? The answer is yes and no. There ARE differences between the .223 Remington as shot in civilian rifles and the 5.56x45 in military use. While the external cartridge dimensions are essentially the same, the .223 Remington is built to SAAMI specs, rated to 50,000 CUP max pressure, and normally has a shorter throat. The 5.56x45 is built to NATO specs, rated to 60,000 CUP max pressure, and has a longer throat, optimized to shoot long bullets. That said, there are various .223 Remington match chambers, including the Wylde chamber, that feature longer throats. Military 5.56x45 brass often, but not always, has thicker internal construction, and slightly less capacity than commercial .223 Rem brass.
Should you be worried about shooting 5.56x45 milspec ammo in a .223 Remington? The answer really depends on your chamber. 5.56 x45 ammo is intended for chambers with longer throats. If you shoot hot 5.56x45 ammo in short-throated SAAMI-spec chambers you can encounter pressure issues. The new long-throated 'Wylde' chamber allows safe use of military ammo. Wylde chambers are quite common in Rock River guns. Other manufacturers, such as Fulton Armory, offer modified "match chambers" with extended throats that allow safe use of 5.56x45 ammo in .223 Remington rifles. For a complete discussion of the .223 Rem vs. 5.56x45 question, read this Tech Notice from Winchester, and the GunZone Commentary by Dean Speir. Without belaboring the point, we'll repeat the official SAAMI position: "Chambers for military rifles have a different throat configuration than chambers for sporting firearms which, together with the full metal jacket of the military projectile, may account for the higher pressures which result when military ammunition is fired in a sporting chamber. SAAMI recommends that a firearm be fired only with the cartridge for which it is specifically chambered by the manufacturer."
So I don't leave anything out, I'm going to start this off pretending nobody knows nothing'. A rifle chamber is a hole cut in the breech end of a barrel so a round of ammunition will fit. It's a lathe operation. A "chamber reamer" is the tool that cuts this hole and it is shaped the same as a cartridge case with at least part of a bullet stuck in it. The reamer is going to cut out the case body and shoulder silhouette, the case neck, and then extend into the bore to form a bullet profile silhouette. It's here, the bullet profile area, where major tooling differences exist. There are a lot of different .223 Remington reamers. The two most commonly used in factory-done guns are at opposite ends of this universe--one is the shortest, and one is the longest.
Let's look closer. What I called the "bullet profile area" is technically called a "leadee." We can also call it the "throat." Inside the chamber, the distance between the end of the case neck and the first point cut into the rifled portion of the barrel coinciding with the barrel's land (rifling) diameter is the preeminent variable determined by the reamer. Land diameter will be the smallest dimension inside a bore. If the first point of full land diameter (usually 0.219" in a 224-caliber barrel) is farther from the end of the case neck (farther into the bore), then the chamber has a longer leade or throat. The bullet won't contact the lands until, of course, it reaches the point on the bullet that coincides with land diameter. I call this the first point of "major diameter" on a bullet. The effect or influence of this conical space ahead of the case neck is simple: The more space the less pressure, and the more space, the farther the bullet must "jump" until the bullet contacts the lands. Read all that again.
Mole Hill And Mountain Now, SAAMI (Small Arms and Ammunition Manufacturer's Institute) long ago set its standards for .223 Remington based on bolt-action rifles chambered for this round. These bolt rifles were configured for varmiting. There was, of course, originally a military chamber and round in use since the .223 Remington commercial round was renamed from the 5.56x45mm (NATO-spec) cartridge. The SAAMI chamber has a good deal shorter leade or throat than a military NATO-spec chamber. There is material elsewhere addressing the reasons this was a bad idea (SAAMI's bad idea), and it's become an even worse idea because it's never really been adequately explained to the folks--like you and me--who load or purchase ammunition for AR-15s. See, off-the-shelf AR-15s can have either chamber. Even worse, some barrels are not marked and some are improperly marked. Compounding matters (but not necessarily making them worse) is competitive use of AR-15s resulted in even more chambering options, and reamers. These came about when 80-grain bullets became available and immediately popular. The SAAMI chamber was too short and the NATO was too long.
So the rest of this will make sense, the following dimensions are all based on an overall cartridge length that will have a Sierra 80-grain MatchKing bullet just touching the lands when the round is chambered. We don't all shoot Sierra 80-grain MatchKings, and we don't all set them to touch the lands, but most competitive High Power Rifle shooters do both. At the least it's a "standard" that gives us a point to work from. What I call the "Derrick Chamber" (Derrick Martin of Accuracy Speaks) needs an overall cartridge length of 2.442"; the "Wylde Chamber" (for competition-use AR-15 pioneer Bill Wylde) is 2.445"; the "AMU Chamber" (for U.S. Army competition team) is 2.500". There are others, but these are the most popular among competition rifle builders. A SAAMI chamber is normally about 2.410"; a NATO chamber is normally about 2.550". Those are huge differences, and I counted five different reamers just mentioned here.
Shortening the leade area to minimize jump with short bullets that must make magazine-box-length constraints does no favors to the longer, heavier bullets since it requires seating them too far back into the case. That is generally considered a bad thing. Getting the longer, heavier bullets some room to stretch, and the case some room to breathe, means shorter bullets are lacing a jump of relatively epic proportions to get started into the bore. That is generally considered a bad thing.
Bad? Worse? Better? Which generally bad thing is worse, or better? Chambering specification doesn't matter all that much to accuracy, but it can to round performance--not the same thing. The .223 Remington has a short case neck, a small body and, well, it's not the perfect round for 600-yard performance. It is, however, what we have to work with. Making it work its best means giving as much room as reasonably possible to the long bullets. This is done to prevent seating them so far back into the case. We need all the powder capacity we can get. I'm a fan of longer rather than shorter leadee specs. Others disagree. There's no answer that can't be argued beyond an average man's concept of a "day."
Different Chamber OAL: (2.260 inches max OAL for Mag Load)
SAAMI - 2.410 (223 only)
Derrick Martin Chamber - 2.442 (223 ammo - Special Loads)
Bill Wylde Chamber - 2.445 (223 ammo - Special Loads)
AMU - 2.500 (5.56 - Special Loads)
NATO - 2.550 (5.56 - Special Loads)
Chambers: that all-important point where everything starts The good news is a longer throat doesn't seem to matter to the performance of shorter bullets. That's not to say it couldn't matter, but for it to positively influence groups using, say, a 77-grain Sierra MatchKing, the throat would have to be way shorter than what anyone uses in a High Power chamber. I've jumped to my own conclusion that once bullet-jump exceeds a few thousandths I'm not sure it matters. Jumping .015" isn't going to help much more than jumping .035". Reality is that we're shooting targets for score, and, therefore, we must judge the supposed good or bad effects from compromises by score. It's really common and easy to clean a 300-yard High Power Rifle target, with a high X-count, in a "long" chamber shooting "short" bullets, like the Sierra 77. That target has a 7" 10-ring.
5.56mm NATO vs. SAAMI .223 Remington Pay attention to this! Out of the box, chances are an AR-15 will have either a SAAMI or a NATO chamber. There are huge differences. Specifically, 5.56x45mm NATO specs call for a longer leadee than SAAMI defined for commercial .223 Remington (which was originally determined for bolt-action rifles). Leadee is the portion of the barrel ahead of the chamber where the rifling has been conically removed to allow room for the seated bullet. A shorter leadee raises pressures. Compounding this, military ammunition is nearly always loaded to higher pressures than commercial. Shooting 5.56mm mil-spec ammo in a SAAMI "minimum" .223 Remington chamber can jump up chamber pressure 15,000 psi, or more. Not all AR-15 barrels are correctly marked, and some aren't marked at all. Know by asking the manufacturer, or just shoot ".223 Remington" ammunition and don't worry. Know also before selecting loading data. If loads were worked up in a NATO chamber (Colt HBAR, for instance), they will be overpressure if used in a SAAMI chamber.
Not Too Tight Don't get too "precise" in chambering an AR-15. Leave that to the single-shot crowd who tediously and continuously prepare their ammunition. Don't ask for a headspace that's too tight (short), a neck diameter that's too small, a body area that's too close to new case dimensions, or a leade that's too short. The limits, to me, are found in looking at the ammunition you want to be able to fire in the rifle, and also what you want to obligate yourself to in the way of making dimensional corrections in your hand loading process. I believe that an AR-15 chamber should be able to accommodate just about any ammunition. The good news is that you won't see any difference in on-target performance. This rifle can't show it. The main effect of "matching" ammo specs and chamber specs is longer case life and less dimensional change firing to firing. Have the chamber polished and keep it clean!