Barrel twist rates and chambering nomenclature confuse many AR-15 shooters, but understanding how different twist rates affect different bullets and how chambers differ can help you shoot much more accurately.
A barrel twist rate reflects the distance a bullet must travel through to make one complete revolution or “turn.” A 1-9 is read as “one-turn-in-9-inches.” The lands must apply enough spin to stabilize the bullet. Although it’s really the bullet length that determines the necessary twist, not its weight, minimum twist rates are usually associated with bullet weights. I’ll explain the reason for this.
Some bullet designs result in a longer length than others at the same grain weight. A 70-grain “VLD” (Very Low Drag) design is an example. This bullet configuration is considerably longer than, say, a 68-grain Hornady. The 1-9 will stabilize the Hornady but not always the VLD. VLD bullets are for competitive shooters who handload, pretty much, so you’re not likely to see them in use otherwise (no factory loadings).
A 1-12, which is increasingly unusual (surprising because it was the “original” mil-spec twist), will adequately spin anything up to 60 grains. A 1-9 will stabilize all commercially-loaded bullets I know of up to 70 grains. Any bullet heavier than that needs the 1-7 (1-8 is fine too, and increasingly common in factory-made uppers). The most common need for a 1-8 or 1-7 is commercial ammo with 75- or 77-grain bullets. A 1-9 will not stabilize those.
Advice? When it’s a choice, choose a faster twist. A 1-7 provides enough flexibility to launch anything up to and including an 82-grain specialty bullet. Accuracy differences shooting lighter bullets through a faster twist won’t be noticed. It’s not Benchrest. Now, don’t fire very light bullets through a 1-7. I’m talking about the 35- to 45-grain kind. Too much rotation at high velocity can create jacket damage, which can lead to “blown up” bullets. (Bullets “blow up,” by the way, from the back forward. Won’t hurt your rifle; they just won’t make it to the target.)
That was easy enough. Now, another question is, “which chamber”? I tend to always risk being too simple to start, because it usually makes it easier to understand the overall better, so, a rifle chamber is a hole cut into the breech-end of a barrel so a cartridge 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 profile at its end. This tool 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 outline. It’s the bullet-profile area where we find reamer tooling differences. There a few different takes on reamers in use by custom builders (they like to tinker), but the two most common used for factory-done barrels are at polar extremes, the shortest, and the longest: .223 Remington and 5.56x45mm NATO. The .223 Remington is sometimes called a “SAAMI” or “SAAMI Minimum.” It’s also commonly denoted as a “match” chamber in factory-built rifles.
The “bullet profile area” I mentioned is rightly called a “leade,” and more commonly the “throat.” The space between the end of the case neck and the first point that coincides with land or rifling diameter (usually 0.219 inches in a .224 caliber) is the influential variable set by the reamer. The farther in this is, the “longer” the throat. Of course, the bullet won’t contact the lands until it reaches the point on the bullet that equals land diameter. The influence the location of this point has is simple: the greater space, the less pressure. And vice-versa. Also, of course, the greater the space, the greater the “jump” the bullet has to make to engage the rifling.
NATO has a whopping lot more space. The reason for the difference in the SAAMI and the NATO came long years ago. SAAMI set its standards for commercial .223 Remington based on bolt-actions configured for varminting. There was a military chamber, and round, in use since the .223 Remington commercial round was renamed from the 5.56x45mm (NATO-spec) cartridge. As civilian use of the AR-15 became more and more popular, commercially made barrels in AR-15s may have either chamber. That’s not a problem as long as you know which you have. To make things worse, some barrels are not marked and some are improperly marked. If you see a configuration advertised as having “match” barrel, it may very well have the SAAMI chamber. If you want to have an easy life with your rifle, get yourself a NATO chamber. You can shoot any commercially loaded ammo in that one, and also mil-spec, plus surplus. If you need to know, and you do need to know, you have to ask.
The short throat in a SAAMI .223 Remington means higher pressure. Combined with the fact that NATO-spec ammo is loaded to higher pressures than commercial .223, firing mil-spec ammo in a “minimum” chamber can increase pressure to the tune of 15,000 psi. That’s enough to have case failures, and conceivably receiver cracks. You have to know. If you don’t know, use only ammunition marked “.223 Remington.” Know also before trying any recipe found in a reloading manual. If the loads were tested through a NATO chamber (Colt HBAR, for instance), they will be over-pressure in a SAAMI. It’s also possible, due to differences in bullet profiles, for a NATO bullet to “stick” into the lands in a SAAMI-chambered barrel when the round is chambered; that causes even higher pressure.
Information in this article was adapted from “The Competitive AR15: The Ultimate Technical Guide,” published by Zediker Publishing.
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