A 1/12 would easily handle any 60-64 grain lead core flat base bullet such as the 60 gr. Nosler Partition, Hornady SP or HP, 63 gr. Sierra semi spitzer or 64 gr. Winchester Power Point.
I would stay away from the longer boat tail, plastic tipped or monometal bullets over 60 grains in a 1/12 twist, although some of them might work better in a .22/250 than in a .223 due to the higher velocity the .22/250 achieves.
