Here's the reply I posted at Shooter's Forum:
All H&R 22-250 barrels have 1:14" twist and it is the standard for the industry, Shilen recommends 55gr as the heaviest .224" bullet for a 1:14" twist, but not all bullets are equal in that respect, some bullets like the 64gr Win PP and 63gr Sierra Varminter #1370 may work in them since they're short for their weight, they work fine in 1:12" .223 barrels, dunno if they'd work at 22-250 velocities in the 1:14" barrel tho, but if I wanted a heavier bullet, I'd sure try them first, a flat base bullet would be more likely to work than a boattail too. The Lyman 48th shows data for the 60gr Hornady HP #2275 and the 63gr Sierra SMP #1360 in their 1:14" 22-250 test gun.
Tim