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Bob's Boattails in Slower Twists

Started by rsterne, August 12, 2019, 11:45:09 AM

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There has been anecdotal evidence since I introduced the idea of my Rebated Boattail bullet designs that the minimum twist rate calculated by the Kolbe Twist Calculator was conservative.... ie shorter/faster than required for stability.... The first hard evidence of this has come to light by two different shooters successfully using my .308 cal 154 gr. BBT in a 14" twist.... Here is the Kolbe chart of twist rate vs. velocity for the 154 gr. BBT for a Stability Factor of 1.5 (recommended)....

As you can see, an 11" twist is optimum at 1000 fps (the velocity I usually use for airgun bullets).... The Kolbe calculator allows you to input your twist rate, and generate a chart showing what the SF is at various velocities.... Here it is for the 154 gr. BBT in a 14" twist....

According to that chart, that bullet would be unstable at between 950 fps and Mach 1.... and the SF would be just over 1.1 (barely stable) at 800 fps.... Ricky B (sixshootertexan) has many targets at 100 yards showing good groups with this bullet (and nice round holes in the target) at a muzzle velocity of about 815 fps from a 14" twist TJ's barrel.... Another shooter has been using it in a Texan (also a 14" twist) and reporting good accuracy as well, this time at 935 fps.... One of two things appears to be happening here.... Either the bullet works OK when the stability is very marginal (SF just over 1.0).... OR the Kolbe calculator is underestimating the stability because of the rebated boattail (it assumes a smooth, conventional boattail).... Either way, the bullet is usable in a slower twist rate than the Kolbe calculator comes up with.... How much slower, we don't yet know.... We also don't know what would happen if they were pushed closer to Mach 1.... If anyone can test them at 1050-1100 fps, that would be very valuable data to have....

I have several other BBTs of exactly the same proportions as the .308 cal 154 gr. (which is 3.0 calibers in length).... Since the twist rate (in inches) is proportional to the caliber, they should also be usable in a slower twist rate than what Kolbe says is optimum.... The bullets in question, and the equivalent twist rate to a 14" with the .308 cal 154 gr. are:

7mm (.284 cal) 121 gr.... 13" twist instead of 10"
.257 cal 90 gr.... 12" twist instead of 9"
.224 cal 60 gr.... 10" twist instead of 8.5"

While I can't draw firm conclusions for BBTs of other proportions, it certainly points the way to being able to use them in slower twist rates than the Kolbe Twist Calculator predicts as well.... As always, data trumps calculations.... so if any of you have experience using BBTs in slower twist rates than we "thought" they should use.... please post in this thread....

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