BoydAllen
Gold $$ Contributor
I have seen where some people have asked about twist rate for the new Sierra 6mm 110 gr. SMKs. I thought that I would add a little info on the subject. A friend has built a rifle that he uses to shoot at plates out to over 1,100 yards with some friends. He builds his own rifles, and has several that he uses for this informal shooting. One is chambered for a tight neck 6mm Rem. AI. It had previously been used with Berger VLDs and the barrel was an 8 twist. Like everyone else, my friend was interested in the new 110s because of the high BC. At first he tried the 8 twist barrel and although the bullets went through the target point first, the accuracy was very mediocre, nowhere near the bullet hole cutting bullet hole that he expects (at 100 yards off of a good bench, shooting over flags) , so he ordered a 7.5 twist (Bartlein I believe.) He worked up loads with the new bullets and tried a number of powders, but that is not my subject. Finally, he came up with a load that shoots very well. The velocity is 3,180 FPS. Now we get to the good part, I think that given the accuracy that he is getting both at 100 yards and at long distance, we can conclude that the spin rate is a good one.
Here is the math. We take the velocity, which is in feet per second and multiply that by 60 to get feet per minute. Then we multiply by 12 to get inches per minute, and divide by the twist rate, 7.5 to get RPM. In his case that works out to 305,280 RPM. Of course smaller cases will not be able to match his velocity, so you may need to do some testing to see what your speed is, by shooting some 110s with your old twist rate for your caliber too see what your velocity is. For example, if you are shooting at 2,800 fps with the 110s your velocity is 88% of my friend's, so to get to the same bullet RPM you would need a faster twist (lower twist number). Doing the math 7.5 times .88 equals 6.6. To check ourselves we redo the original calculation using 2,800 and 6.6. 2,800 x 60= 168,000 x 12= 2,016,000 divided by 6.6 = 305,454. (Chalk up the difference to rounding.) My point is that Sierra's twist requirement of 7 does not seem to be as conservative as it may seem at first glance.
Here is the math. We take the velocity, which is in feet per second and multiply that by 60 to get feet per minute. Then we multiply by 12 to get inches per minute, and divide by the twist rate, 7.5 to get RPM. In his case that works out to 305,280 RPM. Of course smaller cases will not be able to match his velocity, so you may need to do some testing to see what your speed is, by shooting some 110s with your old twist rate for your caliber too see what your velocity is. For example, if you are shooting at 2,800 fps with the 110s your velocity is 88% of my friend's, so to get to the same bullet RPM you would need a faster twist (lower twist number). Doing the math 7.5 times .88 equals 6.6. To check ourselves we redo the original calculation using 2,800 and 6.6. 2,800 x 60= 168,000 x 12= 2,016,000 divided by 6.6 = 305,454. (Chalk up the difference to rounding.) My point is that Sierra's twist requirement of 7 does not seem to be as conservative as it may seem at first glance.
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