Probably the 338 if looking for subs only, else leave it 300 BO. I have shot 185 gr cast from the 10" 1:8 BO pistol with good results but not subsonic. Use 2400 powder. Got 2 1/2 # of it left. Took a bunch of kids (8-10 yrs old) to Ar once, they had fun with the cast loads, shot up all my ammo.