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I am building a 416 Taylor and was trying to determine what is the best twist rate. I expect to shoot from 300 gr to 400 gr. The rifle is just a project at this time with no intended use, DG or otherwise, however, I would not want to limit the ability of the rifle to handle a selection of bullet weights because of twist rate. The info I have suggests 1 in 14" is the standard. | ||
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I would get a twist rate faster than 1:14 to enable using the long monometal bullets. 1:12 would be good, 1:10 would be acceptable to me. I don't think either will give you a big pressure spike upon engraving. For a 400 gr bullet at 2400 fps the KE(translational) is roughly 5100 ft-lbs. The rotational KE is 22 ft lbs for a 1:14 twist. Increasing the twist to 1:10 doubles the rotational KE, but it is still only 44 ftlbs, or less than 1% of the total energy. This means that it will cost you at most 5 fps of bullet velocity to keep the same total energy when you go to the 1:10. Does 2395 fps sound OK?  Don_G ...from Texas, by way of Mason, Ohio and Aurora, Colorado! | |||
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Excellent advice Don. Please go to the 500 A-Square thread on the Big Bores forum and enlighten the heavy darkness there.  | |||
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I understand that twist rate is used to stabalize differrent configuration of bullets, but I have never heard of rotational KE as it applies to twist. Would you mind explaining how I interpret rotational KE or where I could go to read up on it. Thanks. | |||
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I have been in the 416 Taylor process for a while and I went with 1:16, which is closer to the original 416 Rigby twist rate. Red | |||
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I don't know if you can find it but I think I posted the formula for calculating a rotational KE a year or so ago. Getting the long monometal bullets into a typical Taylor action and still get 2400 fps is going to take some doing. Douglas makes 1 in 12 and 1 in 14 twists. The 1 in 14" gave me good results with 400 gr. Speer AGS soilds on elephants. My other Taylor has a 1 in 12, but I doubt that it will make much difference other than on the internet. If you can't find the rotational energy stuff I can e-mail it to you. ------------------------------- Will Stewart / Once you've been amongst them, there is no such thing as too much gun. --------------------------------------- and, God Bless John Wayne. NRA Benefactor Member, GOA, N.A.G.R. _________________________ "Elephant and Elephant Guns" $99 shipped “Hunting Africa's Dangerous Game" $20 shipped. red.dirt.elephant@gmail.com _________________________ Hoping to wind up where elephant hunters go. | |||
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The equations are: KE(translational) = 1/2 Mass*Velocity*Velocity For bullet weight in grains and speed in Feet/second, KEt = (Bullet Weight)*Velocity*Velocity/450800 Rotational is similar, but first you have to calculate the mass moment of inertia (MMI). Approximating a bullet as a column: MMI = 1/2*Mass*Radius*Radius Then the rotational kinetic energy is: KEr = 1/2*MMI*W*W, where W is the rotational velocity in radians/second (W = revolutions/second * 2 * Pi) or, W = 24*Pi*V/T, where T = Twist rate in inches, and V is again Velocity in feet/second. Boiling all that down, KEr = (Bullet Weight)*Caliber*Caliber*Velocity*Velocity/(Twist*Twist*91351) Dago Red, I think the 1:16 is a bit slow for the monometal bullets. I'd stick to traditional lead-based bullets at that twist rate at Taylor velocities. The monometal solids might be accurate, but may not track straight inside the carcase. Don_G ...from Texas, by way of Mason, Ohio and Aurora, Colorado! | |||
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Don_G, Calculating the rotational KE is not difficult given the formulas you have provided. Thanks. But being a layman what do I do with those figures? Do they represent anything other than what it takes to stabalize a bullet of a certain weight and configuration? Or is there something else that I am missing in bullet performance? | |||
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The THEORY is that if the bullet is spinning faster that it will more likely be stable once it gets inside an animal. The rotational KE really doesn't have anything to do with it except that the faster spin has a higher rotational KE. Rotational KE isn't going to kill anything. But since the bullet is going to stop spinning at some point if it stays inside the aminal, I find it somewhat doubtful that a buff is going to know the difference. If you stick with the relatively short lead core bullets it is all paper arguments anyway. ------------------------------- Will Stewart / Once you've been amongst them, there is no such thing as too much gun. --------------------------------------- and, God Bless John Wayne. NRA Benefactor Member, GOA, N.A.G.R. _________________________ "Elephant and Elephant Guns" $99 shipped “Hunting Africa's Dangerous Game" $20 shipped. red.dirt.elephant@gmail.com _________________________ Hoping to wind up where elephant hunters go. | |||
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There are differing opinions on whether a fast twist rate helps stability in the terminal ballistics phase of flight (inside the critter). I think it helps a lot, but that's just an opinion. The only reason to calculate the rotational kinetic energy is to point out that it is an insignificant amount of energy compared to the translational KE (that's the KE that is delivered to the target). The total energy of the bullet is the sum of the KEt and the KEr. This total energy is derived from the integral of the pressure that pushes the bullet down the barrel. There is a max pressure that is safe for any given cartridge and rifle - this limits the total energy available. Since the KEr is small compared to the KEt, it does not rob any appreciable power from the bullet, nor does it cause a pressure "spike". The implication is that it does not cost you much in pressure or translational KE to get whatever benefits there may be from the added stability. My thinking on the energy budget is: "It costs me less than a penny on the dollar, and it might help a whole lot." Don_G ...from Texas, by way of Mason, Ohio and Aurora, Colorado! | |||
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Well said, Don. | |||
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