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Does barrel length and rifling have much affect on transonically stabilizing a bullet? I have a friend who builds custom tactical rifles and he made one with a barrel that was about three feet long. He was trying to explain it all to me but I was in a hurry that day and I didn't quite get all of it. Just curious.
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Most serious 1K competitors using the 308 will use a 28-30 inch barrel. They will hand load their own ammunition and will push pressures as high as they can without affecting group size. The idea is to achieve high velocity to minimize wind drift and drop variations and to stay above Mach 1.2.
Barrel twist is not especially important to transonic stability above and beyond regular stability concerns. Bullet shape is important. Shallow boat tail angle and no meplat or very small meplat, and short bearing surface seem to be important. Interestingly, the old black powder buffalo sharps aficionados shoot cast lead bullets very accurately far beyond the transonic zone.
As far as the Swat study I have not shot any factory loaded rifle ammo in three decades so I just don't have any knowledge base concerning burning rates of the powder used.