The first thing I would do is check my stock screws and make sure they are tight. Same with the sights (or scope mount and ring screws if you are using a scope). If the gun is new, I would put at least 20 .50 lead conicals through it, without cleaning between shots, to sort of lap in the barrel. Also, your gun just might not like the powder/bullet combination. Try two Pyrodex pellets with the 250 grain Shockwave.