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Rifles, Reloading, Optics, Equipment
AR15/10 Rifles
Help with AR15 Varmint Build
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<blockquote data-quote="budlight" data-source="post: 1034282" data-attributes="member: 2939"><p>This is not entirely a true statement. Twist rates create a bullet RPM with velocity as a factor. The higher the velocity for a given twist rate the higher the RPM. Shorter length barrels attain lower velocity for a given combination. In testing with a chrono and 1:7 we found that 55 grain .223's when shot at sub 3000 fps could actually stabilize. So 14.5, 16, and possibly 18 inch barrels using factory low powered rounds. </p><p></p><p>Higher than 1:9 is really a dumb move if you intend to shoot 62 grains or less. The higher the twist rate the higher the resistance to a bullet traveling down a barrel. So 1:8 and 1:7 will have less velocity. </p><p></p><p>So don't lock yourself into using low velocity 55 and 62 grain bullets and or only heavier bullets. Spinning a bullet faster than necessary can amplify any inconsistency in the bullet.</p><p></p><p><strong></strong></p><p><strong>Quote:</strong></p><p><strong></strong></p><p><strong>1-in-9 Inches</strong></p><p> <strong>This is the beginning of the road for the shooter wanting to take advantage of the heavy bullet trend. The 1-in-9 is a great compromise twist rate—not too fast to cause problems with the 55-grain Bullets, but fast enough to stabilize all but the heaviest bullets under most circumstances. This twist will stabilize most traditional bullets up to 75-grains, and monolithics up to 70-grains—but they do so right at the edge of the envelope so not all rifles will do it. My personal 16-inch Rock River Arms carbine with a 1-in-9 twist does fine with ASYM's Tactical Match Grade 77-grain OTM load, but has shown signs of instability with handloads using the 70-grain Barnes TSX—unless the bullet is pushed to maximum velocity. With longer barrels and the commensurate faster velocities, this twist can be more forgiving.</strong></p><p><strong></strong></p><p><strong>1-in-8 Inches</strong></p><p> <strong>For a 16-inch general-use carbine, the 1-in-8 twist is about as versatile as it gets. This twist rate will comfortably stabilize bullets up to 80-grains, and the excellent 75- and 77-grain bullets also work great at a wider spectrum of velocities—which means barrel length isn't critical. My 3-gun rifle, built by my friend Iain Harrison, wears an 18-inch, 1-in-8 twist White Oak Armament barrel and shoots just about anything well.</strong></p><p><strong></strong></p><p><strong>1-in-7 Inches</strong></p><p><strong>This is the twist chosen by the military since the switch was made to the M16A2—and the 62gr. M855 cartridge—in the 1980s. This twist is found on the M4 carbine, the M16A4, the Mk12 Special Purpose Rifle and even the HK416. Its ability to stabilize tracer rounds in-flight is one of the reasons that the military chose this twist rate.</strong></p><p><strong></strong></p><p><strong>This barrel will stabilize bullets of up to 90 grains, and can handle the 70- to 77-grain bullets at just about any velocity, which makes it well suited for carbines with very short barrels. If you want a Mil-Spec clone, the 1-in-7 twist is the way to go.</strong></p><p><strong></strong></p><p><strong>I currently own two AR-15s with 1-in-7 twist rates—a 10.3-inch Mk18 Mod. 1 and an 18-inch Mk12 Mod. 1—both made by Monty LeClair at Centurion Arms. To illustrate how velocity and twist rates can be the ying and the yang of bullet stability and accuracy, I fired a variety of loads side-by-side from the two rifles. The 10.3-inch SBR shoots anything from 55-grain FMJ to 77-grain OTM with excellent accuracy, while the 18-inch Mk12 is scary accurate with the 70-, 75- and 77-grain bullets—but won't shoot 55-grain or 62-grain FMJs worth a ****.</strong></p><p><strong></strong></p><p><strong>Matching the rifling twist in your rifle or carbine to the appropriate ammunition won't guarantee great accuracy, but it will ensure the bullet is properly stabilized in flight. On the other hand, using a bullet that's too heavy for your barrel's twist is a virtual promise of poor accuracy and ineffective terminal performance. If you're struggling with the accuracy of your modern sporting rifle, be sure you've properly matched your ammunition to the barrel's twist.</strong></p></blockquote><p></p>
[QUOTE="budlight, post: 1034282, member: 2939"] This is not entirely a true statement. Twist rates create a bullet RPM with velocity as a factor. The higher the velocity for a given twist rate the higher the RPM. Shorter length barrels attain lower velocity for a given combination. In testing with a chrono and 1:7 we found that 55 grain .223's when shot at sub 3000 fps could actually stabilize. So 14.5, 16, and possibly 18 inch barrels using factory low powered rounds. Higher than 1:9 is really a dumb move if you intend to shoot 62 grains or less. The higher the twist rate the higher the resistance to a bullet traveling down a barrel. So 1:8 and 1:7 will have less velocity. So don't lock yourself into using low velocity 55 and 62 grain bullets and or only heavier bullets. Spinning a bullet faster than necessary can amplify any inconsistency in the bullet. [b] Quote: 1-in-9 Inches This is the beginning of the road for the shooter wanting to take advantage of the heavy bullet trend. The 1-in-9 is a great compromise twist rate—not too fast to cause problems with the 55-grain Bullets, but fast enough to stabilize all but the heaviest bullets under most circumstances. This twist will stabilize most traditional bullets up to 75-grains, and monolithics up to 70-grains—but they do so right at the edge of the envelope so not all rifles will do it. My personal 16-inch Rock River Arms carbine with a 1-in-9 twist does fine with ASYM’s Tactical Match Grade 77-grain OTM load, but has shown signs of instability with handloads using the 70-grain Barnes TSX—unless the bullet is pushed to maximum velocity. With longer barrels and the commensurate faster velocities, this twist can be more forgiving. 1-in-8 Inches For a 16-inch general-use carbine, the 1-in-8 twist is about as versatile as it gets. This twist rate will comfortably stabilize bullets up to 80-grains, and the excellent 75- and 77-grain bullets also work great at a wider spectrum of velocities—which means barrel length isn’t critical. My 3-gun rifle, built by my friend Iain Harrison, wears an 18-inch, 1-in-8 twist White Oak Armament barrel and shoots just about anything well. 1-in-7 Inches This is the twist chosen by the military since the switch was made to the M16A2—and the 62gr. M855 cartridge—in the 1980s. This twist is found on the M4 carbine, the M16A4, the Mk12 Special Purpose Rifle and even the HK416. Its ability to stabilize tracer rounds in-flight is one of the reasons that the military chose this twist rate. This barrel will stabilize bullets of up to 90 grains, and can handle the 70- to 77-grain bullets at just about any velocity, which makes it well suited for carbines with very short barrels. If you want a Mil-Spec clone, the 1-in-7 twist is the way to go. I currently own two AR-15s with 1-in-7 twist rates—a 10.3-inch Mk18 Mod. 1 and an 18-inch Mk12 Mod. 1—both made by Monty LeClair at Centurion Arms. To illustrate how velocity and twist rates can be the ying and the yang of bullet stability and accuracy, I fired a variety of loads side-by-side from the two rifles. The 10.3-inch SBR shoots anything from 55-grain FMJ to 77-grain OTM with excellent accuracy, while the 18-inch Mk12 is scary accurate with the 70-, 75- and 77-grain bullets—but won’t shoot 55-grain or 62-grain FMJs worth a ****. Matching the rifling twist in your rifle or carbine to the appropriate ammunition won’t guarantee great accuracy, but it will ensure the bullet is properly stabilized in flight. On the other hand, using a bullet that’s too heavy for your barrel’s twist is a virtual promise of poor accuracy and ineffective terminal performance. If you’re struggling with the accuracy of your modern sporting rifle, be sure you’ve properly matched your ammunition to the barrel’s twist.[/b] [/QUOTE]
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