Rick,
The Berger twist calculator, as I have pointed out a few times before on the Campfire, is based on a formula worked out by the late Don Miller, as are most twist formulas these days. I knew Don fairly well, since we both liked to fool with ballistic numbers, and after he derived the basic formula, he came to the conclusion that plastic tips shouldn't be counted in the length of the bullet. Or at least the majority of their length shouldn't, because they weigh so little.
The Berger version of the Miller formula doesn't take this into account, the reason it predicts a number of plastic-tipped bullets won't stabilize in standard twists under certain conditions, even though nobody's ever had a problem with 'em. A good example is the 40-grain V-Max in the .204 Ruger with the standard 1-10 factory twist.
The JBM Ballistic twist formula includes a place to list the length of plastic tips, and works more realistically with such bullets. It indicates a 1-10 twist is plenty for the 150 .270 ABLR, which has been my experience in more than one rifle.
