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AR-15 Barrel Question
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<blockquote data-quote="dennishoddy" data-source="post: 3164449" data-attributes="member: 5412"><p>The used of a contricity tool will greatly reduce the "wobble".</p><p></p><p><img src="https://media.mwstatic.com/product-images/880x660/Primary/220/220200.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p></p><p>When looking to buy a new rifle or build one of your own, one of things to look at is rifle twist. This is a characteristic of the barrel that will affect how your bullets perform down range and, to some degree, will dictate what rounds should be fired out of that weapon. Rifle twist is expressed as a ratio. An example would be a 1/7 or 1:7 twist. This means that for every seven inches of barrel, there will be one complete twist or rotation of the rifling, which imparts spin onto the bullet. Without getting too in depth about bullet types, this should explain the basics of how rifle twist influences bullet flight and what a buyer should look for to fit their own needs.</p><p></p><p>The rifling of the barrel causes a bullet to spin. This spin is directly related to how “tight” or “loose” the rifling in the barrel is. Faster spins are good for longer bullets as they need more spin to stabilize their center of gravity. Often times, shooters choose tighter twist rates for heavier bullets. This can make shooters think that it is the weight of the bullet that requires a certain twist rate. This is not (entirely) true. To make a bullet heavier, the bullet is made longer to add material without increasing caliber. It is this <em>length</em> that needs the twist rate.</p><p></p><p><img src="https://blog.uspatriottactical.com/wp-content/uploads/Rifling-300x300.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" />Early M16s used a 1:14 twist rate as they fired light and short bullets. This rate was very limited in what it could do; not even the 55 grain rounds used were spinning fast enough. The military then moved up to 1:12 twist that put a better spin on the bullet. As heavier rounds came out, the twist rate became tighter and tighter. The 1:9, the military found, was good still slow enough for the 55 grain rounds, but it was fast enough for 75 grain bullets as well. As barrels became shorter however, it was noted that even tighter twist rates were needed. This was because a bullet needs at least two rotations in the barrel to become stable. A 16 inch barrel with a 1:9 twist cannot deliver this, so the twist rate was tightened to 1:8. As barrels became even shorter and bullets became heavier, twist rates were again increased. During testing however, it was noted that bullets that twisted too fast could tear themselves apart if the copper jacket had any imperfections. For this reason, the 10.5 inch barrels on AR15s still use a 1:7, sacrificing a full two rotations in order to not over spin the round and cause it to break apart.</p><p></p><p>With that little history lesson, we can start to see what is needed for our rifle barrels. If you plan to use a rifle for varmint hunting loaded with 45 grain rounds, you are going to want a slow twist rate. If the purpose of the rifle is to have heavy hitting rounds at 300 meters and will be loaded with 70 grain rounds, a twist rate of 1:7 will do better. By matching the proper twist rate to the proper rounds, you can ensure that you will fire stable rounds that will give the most accuracy your rifle has to offer.</p><p></p><p><a href="https://blog.uspatriottactical.com/rifle-twist-101-basic-understanding/" target="_blank">https://blog.uspatriottactical.com/rifle-twist-101-basic-understanding/</a></p><p></p><p><strong>Bullet RPM Formula just in case you were curious.</strong></p><p>Here is a simple formula for calculating bullet RPM:</p><p></p><p><strong>MV x (12/twist rate in inches) x 60 = Bullet RPM</strong></p><p></p><p><strong>Quick Version: MV X 720/Twist Rate = RPM</strong></p><p></p><p>Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.</p><p></p><p>Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM.</p><p></p><p><a href="http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/" target="_blank">http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/</a></p><p></p><p>I'd always heard of bullets coming out of the barrel and then a grey puff, and didn't really believe a bullet could come apart, but they really can. Ultra high velocity rounds only.</p></blockquote><p></p>
[QUOTE="dennishoddy, post: 3164449, member: 5412"] The used of a contricity tool will greatly reduce the "wobble". [IMG]https://media.mwstatic.com/product-images/880x660/Primary/220/220200.jpg[/IMG] When looking to buy a new rifle or build one of your own, one of things to look at is rifle twist. This is a characteristic of the barrel that will affect how your bullets perform down range and, to some degree, will dictate what rounds should be fired out of that weapon. Rifle twist is expressed as a ratio. An example would be a 1/7 or 1:7 twist. This means that for every seven inches of barrel, there will be one complete twist or rotation of the rifling, which imparts spin onto the bullet. Without getting too in depth about bullet types, this should explain the basics of how rifle twist influences bullet flight and what a buyer should look for to fit their own needs. The rifling of the barrel causes a bullet to spin. This spin is directly related to how “tight” or “loose” the rifling in the barrel is. Faster spins are good for longer bullets as they need more spin to stabilize their center of gravity. Often times, shooters choose tighter twist rates for heavier bullets. This can make shooters think that it is the weight of the bullet that requires a certain twist rate. This is not (entirely) true. To make a bullet heavier, the bullet is made longer to add material without increasing caliber. It is this [I]length[/I] that needs the twist rate. [IMG]https://blog.uspatriottactical.com/wp-content/uploads/Rifling-300x300.jpg[/IMG]Early M16s used a 1:14 twist rate as they fired light and short bullets. This rate was very limited in what it could do; not even the 55 grain rounds used were spinning fast enough. The military then moved up to 1:12 twist that put a better spin on the bullet. As heavier rounds came out, the twist rate became tighter and tighter. The 1:9, the military found, was good still slow enough for the 55 grain rounds, but it was fast enough for 75 grain bullets as well. As barrels became shorter however, it was noted that even tighter twist rates were needed. This was because a bullet needs at least two rotations in the barrel to become stable. A 16 inch barrel with a 1:9 twist cannot deliver this, so the twist rate was tightened to 1:8. As barrels became even shorter and bullets became heavier, twist rates were again increased. During testing however, it was noted that bullets that twisted too fast could tear themselves apart if the copper jacket had any imperfections. For this reason, the 10.5 inch barrels on AR15s still use a 1:7, sacrificing a full two rotations in order to not over spin the round and cause it to break apart. With that little history lesson, we can start to see what is needed for our rifle barrels. If you plan to use a rifle for varmint hunting loaded with 45 grain rounds, you are going to want a slow twist rate. If the purpose of the rifle is to have heavy hitting rounds at 300 meters and will be loaded with 70 grain rounds, a twist rate of 1:7 will do better. By matching the proper twist rate to the proper rounds, you can ensure that you will fire stable rounds that will give the most accuracy your rifle has to offer. [URL]https://blog.uspatriottactical.com/rifle-twist-101-basic-understanding/[/URL] [B]Bullet RPM Formula just in case you were curious.[/B] Here is a simple formula for calculating bullet RPM: [B]MV x (12/twist rate in inches) x 60 = Bullet RPM[/B] [B]Quick Version: MV X 720/Twist Rate = RPM[/B] Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel. Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM. [URL]http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/[/URL] I'd always heard of bullets coming out of the barrel and then a grey puff, and didn't really believe a bullet could come apart, but they really can. Ultra high velocity rounds only. [/QUOTE]
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