Well, generally when I take a 'silly' gun out gopher hunting, I use my regular hunting/practice loads in it. This way, you're also getting practice with the same setup you'd use for big game. Although I did once cook up a 3300 fps load for my 308 using 85 grain XTP bullets intended for the 32 ACP. I just run them through a cast bullet sizing die to swage them down a couple thou.
I'll tell ya, though, a 338 shooting 225 grain interbonds really messes a gopher up.