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Old November 29, 2019, 05:42 PM   #13
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 17,027
Ballistics

Mike38,

In theory, you should be more stable reversing an SWC because it puts the bullet's center of pressure behind its center of mass, adding a little static stability and causing the bullet to require less twist to stabilize. Indeed, Finnish small game hunters have been known to make their subsonic loads from boattail bullets loaded backward. The practical problem is that cast and swaged lead bullets are not made with equal surface symmetry to that of jacketed bullets. As a result, when the back edge of the bearing surface clears the muzzle, muzzle blast gasses playing off the tapered sides of the nose, directed by the crown, can introduce lateral drift and increase initial yaw. Most guns (with a good, symmetrical crown) can shoot flat bullet bases better than boattail bullet bases because of the lower muzzle blast effect on yaw.

Quote:
Originally Posted by Mike38
I'm looking for a lighter bullet / less bearing surface to compensate for the slow twist.
If two bullets have the exact same exterior dimensions, the heavier one needs less twist because it has higher angular (rotational) momentum, making it harder to turn off its spin axis. You could prove this to yourself by casting some lead bullets and some from gold ones in the same mold. Because gold is almost 1.7 times denser, you could, driving it to the same velocity, stabilize the gold with just 59% of the twist rate the lead needs.

I know that sounds backward because barrel makers say heavier bullets need more twist, but it's not because of their weight. It's because of their greater length, assuming the bullet construction is the same (not one made of lead and one made of gold). Length trumps weight in the stabilization game. So what you want is a bullet that is shorter. It will be more stable. In the case of a full wadcutter, it will stabilize just fine if the length is right and have a square base for more even muzzle blast effect.

All that said, handgun barrel twist rates tend to be faster than necessary for long-range. Once initial yaw has damped out. But pistols are always dealing with initial yaw at their target ranges.

Apropos of this, I saw an article some years ago in which someone tried different twists in .32 barrels to look at shorter range accuracy with HB wadcutters (less stable than the shorter DE wadcutters, but better able to seal the bore and prevent gas cutting than the DEWC). The standard pitch is 18 3/4", but this guy tried 16", 14", 12" and 10" twist barrels, and IIRC, he found the 12" pitch made the smallest groups. Probably on 25 meter targets.
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