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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
The science to twist rates
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<blockquote data-quote="Gene" data-source="post: 451642" data-attributes="member: 7402"><p>Here is something I have saved, dunno who wrote it:</p><p></p><p>Calculating Bullet RPM from MV and Twist Rate</p><p>The lesson here is that you want to use the optimal RPM for each bullet type. So how do you calculate that? Bullet RPM is a function of two factors, barrel twist rate and velocity through the bore. With a given rifling twist rate, the quicker the bullet passes through the rifling, the faster it will be spinning when it leaves the muzzle. To a certain extent, then, if you speed up the bullet, you can use a slower twist rate, and still end up with enough RPM to stabilize the bullet. But you have to know how to calculate RPM so you can maintain sufficient revs.</p><p>Bullet RPM Formula</p><p>Here is a simple formula for calculating bullet RPM:</p><p>MV x (12/twist rate in inches) x 60 = Bullet RPM</p><p>Quick Version: MV X 720/Twist Rate = RPM</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>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>Implications for Gun Builders and Reloaders</p><p>Calculating the RPM based on twist rate and MV gives us some very important information. Number one, we can tailor the load to decrease velocity just enough to avoid jacket failure and bullet blow-up at excessive RPMs. Number two, it helps us calculate the minimum twist rate needed to stabilize a particular bullet. Although there are other important factors to consider, if you speed up the bullet (i.e. increase MV), you MAY be able to run a slower twist-rate barrel, so long as you maintain the requisite RPM for stabilization. (Note: this is a general principle, but other factors, including bullet shapes, Center of Gravity (CG) and jacket thickness can affect stability). The slower twist-rate barrel may, potentially, be more accurate.</p><p>That said, we note that bullet-makers provide a recommended twist rate for their bullets. This is the "safe bet" to achieve stabilization with that bullet, and it may also</p></blockquote><p></p>
[QUOTE="Gene, post: 451642, member: 7402"] Here is something I have saved, dunno who wrote it: Calculating Bullet RPM from MV and Twist Rate The lesson here is that you want to use the optimal RPM for each bullet type. So how do you calculate that? Bullet RPM is a function of two factors, barrel twist rate and velocity through the bore. With a given rifling twist rate, the quicker the bullet passes through the rifling, the faster it will be spinning when it leaves the muzzle. To a certain extent, then, if you speed up the bullet, you can use a slower twist rate, and still end up with enough RPM to stabilize the bullet. But you have to know how to calculate RPM so you can maintain sufficient revs. Bullet RPM Formula Here is a simple formula for calculating bullet RPM: MV x (12/twist rate in inches) x 60 = Bullet RPM Quick Version: MV X 720/Twist Rate = RPM 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. Implications for Gun Builders and Reloaders Calculating the RPM based on twist rate and MV gives us some very important information. Number one, we can tailor the load to decrease velocity just enough to avoid jacket failure and bullet blow-up at excessive RPMs. Number two, it helps us calculate the minimum twist rate needed to stabilize a particular bullet. Although there are other important factors to consider, if you speed up the bullet (i.e. increase MV), you MAY be able to run a slower twist-rate barrel, so long as you maintain the requisite RPM for stabilization. (Note: this is a general principle, but other factors, including bullet shapes, Center of Gravity (CG) and jacket thickness can affect stability). The slower twist-rate barrel may, potentially, be more accurate. That said, we note that bullet-makers provide a recommended twist rate for their bullets. This is the “safe bet” to achieve stabilization with that bullet, and it may also [/QUOTE]
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