12-05-2016, 02:06 AM
I was reading on the 6.5 SAUM Facebook page the other day and George Gardner says they are moving from 1:8.7 to 1:8. Just one data point. Seems to be the trend, lately.
Bill Alexander always pointed out that the slower the velocity and the longer the bullet, the tighter twist needed. Thus, his 16" tubes are 1:7.5. His 24" were actually 1:9. And, as mentioned, if one plays with Berger's Twist Rate Calculator, one can get good data for any given bullet and velocity and environmental conditions.
Another thing to remember is that Bryan Litz testing showed that each inch difference in twist rate accounts for ONLY 1.33 fps average difference in velocity. So, if your 1:9 twist gives you 2851.33 fps, then your 1:8 will "only" give you 2850 fps. I see no real downside, only pluses, to the tighter twist.
Bill Alexander always pointed out that the slower the velocity and the longer the bullet, the tighter twist needed. Thus, his 16" tubes are 1:7.5. His 24" were actually 1:9. And, as mentioned, if one plays with Berger's Twist Rate Calculator, one can get good data for any given bullet and velocity and environmental conditions.
Another thing to remember is that Bryan Litz testing showed that each inch difference in twist rate accounts for ONLY 1.33 fps average difference in velocity. So, if your 1:9 twist gives you 2851.33 fps, then your 1:8 will "only" give you 2850 fps. I see no real downside, only pluses, to the tighter twist.
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