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GUNS Magazine September 2012 Digital Edition - Page 28
STORY: Glen Zediker tWists & Leades The right barrel. y UPS man flagged me down recently. He told me that (such-and-such) was having a big sale on AR-15 uppers… “Should I get a 9 or a 7…?” He was, of course, referring to the barrel twist rate. My first question back was “What do you want to shoot in it…” I was, of course, referring to ammunition. The twist rate question is fairly easy to answer, although it has more than one part or qualification. It’s the other question that demands a little more attention. That is, “Is it a .223 or NATO chamber?” First, twist rate. A barrel twist rate is expressed as the distance for one complete revolution or “turn” of the rifling. It’s a 360-degree spiral. A 1:9" is read as “1-turn-in-9-inches.” The twist rate must apply enough revs to a bullet to stabilize it. Although it’s really the length of the bullet, not its weight, that determines the necessary twist, it’s usually referenced in bullet weight. The reason I mention this is because there are some bullet designs that create a longer bullet compared to others at the same grain weight. A 70-grain “VLD” (Very Low Drag) design is a good example. These bullets are considerably longer than say, a 69-grain Sierra MatchKing. The 1:9" will stabilize the Sierra but not always the VLD. VLD bullets are for competitive shooters who handload, pretty much, so will not likely be encountered by others. So, the twist rate answers: A 1:9" will stabilize virtually all commercially-loaded bullets up to 70 grains. Anything heavier than that needs the 1:7" (1:8" is fine too, just not as common in factory-built uppers). The most common need for a 1:7" is for those who want to use commercially-loaded ammo with a 75- or 77-grain bullet. A 1:9" will not stabilize those loadings. Practical advice? When unsure, go with the faster twist. A 1:7" twist provides enough flexibility to launch anything up to and including an 82-grain bullet. Accuracy differences with lighter bullets will not be noticeable. By that I mean shooting lighter bullets through a faster twist. The only question might come if someone wants to shoot very light bullets, such as specialty varminting bullets, through a 7" twist. There can be bullet jacket damage, which can lead to “blown up” bullets if we’re shooting something like a 40 grain at max velocity. Again, this is more likely encountered by the handloader and not the loaded ammo purchaser. Now. The chamber question. Overview first. 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 M If you want to shoot 5.56 NATO ammo, make sure you have a NATO chamber. The little “cross” stamp (above, at 12 o’clock) identifies NATO rounds. This is a NATO-spec round (below) fired in a .223 Remington (SAAMI) chamber. Ouch! These are Sierra 80-grain MatchKings seated to engage the lands for a NATO chamber (left) and a SAAMI minimum .223 Remington chamber (right). Whoa. Big difference: more than .150". 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 a number of .223 Remington reamers available and in use by custom builders. The two most commonly used in factory-made guns are at opposite ends of this little universe. One is the shortest—.223 Remington and one is the longest—5.56x45mm NATO. The .223 Remington is sometimes called a “SAAMI” or “SAAMI Mininum.” What I called the “bullet profile area” is technically called a “leade.” We also can 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-caliber bore) is farther from the end of the case neck (farther ahead into the bore), then the 28 W W W. G U N S M A G A Z I N E . C O M • S E P T E M B E R 2 0 1 2