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GUNS Magazine September 2010 - Page 28
UNDeRSTaNDiNG CHaMBeRS Let’s clear our throats. he chamber specifications in any barrel should, one, be known. T Two, if there are options, the more you know the better able you’ll be to get what serves you best. I’m going to start this off assuming no one knows anything about chambers. A rifle chamber is a hole cut in the breech-end of a barrel so a round of ammunition will fit into it. It’s a lathe operation. A chamber “reamer” is the tool that cuts this hole and is shaped the same as a cartridge case with at least a little part of a bullet stuck down into 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 the main tooling differences exist. There are a number of .223 Remington reamers considered and available as “standard” (along with 5.56x45mm NATO). The two most commonly used in factorydone guns are at opposite ends of this little 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 “leade.” We can also call it the barrel “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 that coincides with barrel 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 .219" in a .224" bore) is farther from the end of the case neck (farther ahead 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 affect 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. Chambering specifications, which means the reamer used (and somewhat how it was used), are therefore compromises. Many similar things are. SAAMI (Small Arms and Ammunition Manufacturer’s Institute) set its standards for .223 Remington based on bolt-action rifles chambered for this round. These bolt rifles were configured for varminting. There, of course, was 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. This was a bad idea (SAAMI’s bad idea), and it’s become an even worse idea because it’s rarely been adequately explained to folks like you and me who load or purchase ammunition for AR-15s. See, offthe-shelf AR-15s may have “either” chamber. Even worse, some barrels are not marked and some are improperly marked. Compounding matters (but not necessarily making them worse) Rounds loaded to respect magazine box dimensions are going to jump to engage the lands. The question is how much, and in Glen’s experience it doesn’t really matter. The shape of the bullet matters, and anything with a more “rounded” ogive endures jump better. A Hornady LnL tool will show you the first point on any bullet contacting the lands. use that information to understand your chamber. This tool comes in handy for checking new barrels of uncertain chambering specs. Enough experience and enough notes will show you right off if you’ve got a long or short throat, and how long or how short. 28 is competitive use of AR-15s resulted in even more chambering options and reamers with unique throating dimensions. These came about after 80-grain bullets became available, and then became immediately popular. The SAAMI was too short for these and the NATO was too long. Competitive NRA High-Power shooters talk about chamber “lengths” based on an overall cartridge length that will have a Sierra 80-grain MatchKing bullet just touching the lands when the round is chambered. Not all of us shoot that bullet, and we don’t all set them to touch the lands. However, this has become a “standard” and gives us a way to differentiate chambers with different throats. For instance, the “Wylde Chamber” (named for AR-15 pioneer Bill Wylde) is 2.445". This is a very popular chamber for competition use. A SAAMI-minimum chamber is normally about 2.395", and a NATO chamber is normally about 2.550". Those are huge differences. Back to the start, and the compromises. If we’re shooting different bullets in the same rifle, and these bullets are a good deal different in overall lengths and profiles, there’s no way to get things how we’d like them to work best with each bullet. WWW.GUNSMAGAZINE.COM • SEPTEMBER 2010