My 452 gets a steady diet of Eley SSHP. A little pricier than others mentioned above, but they are absolutely fantastic for squirrel fodder.
I also have a model 455, as well as some of the ancestors of the 452, like the BRNO No.1 and other BRNO models... They too love the Eley SSHP round.
But as others have eluded too, mine will also shoot other brands pretty well, like Wolf MT, SK, CCI SV, and the lowly Federal 510...
dave