If you don't mind the minor hassles of a wildcat, I'd go .20 Vartarg.
The heaviest .224 bullets with decent ballistic coefficients are 40-grain plastic-tips, which can get 3400+ fps in a .221. But .caliber 32's have a similar BC, and in a .20 VT can get around 3700 fps. They shoot noticeably flatter and drift less in the wind than 40's in the .221.
My favorite cartridge on the Fireball case, however, is the .17 Remington Fireball. It really zips both 20's and 25 plastic-tips out there, and while you can spot your own shots with all three rounds, the .17 has the least recoil. But some handloaders have a hard time with the little bullets.
