Only 1 gun fits the bill. The CVA Accura LR. They made a campaign for a bullet that fits 1 gun so far. So 50 cal folks are left out unless you pop the skirt off grind off the post and put it in a sabot but then you need a 1:22 twist barrel.
Plenty of other 1-22 or faster twist 45s out their. Im not so sure a 1-22 is mandatory depending on the length. Ive seen Parker 275gr BEs shoot pretty good in a much slower twist.