I believe the replies so far are both correct. You're pushing them too fast I am guessing. Try dropping your charge 10 grains and see if that does not help.

Also as asked, you are swabbing between shots I hope? Since you're shooting Triple Se7en I would also suggest that before you load the powerbelt, pull the plastic button off the bottom. You will see a lead spike on the conical that the plastic slips back on to. Put a drop of lube, oil, spit, what ever.. It will help them shed the button faster and give you better accuracy.

If you are shooting powerbelts because they are slip fit loading, there are other projectiles out there that are slip fit that will give you better accuracy maybe. They are .503 diameter 460 grain No Excuses Conicals. I shoot them with 85 grains of Triple Se7en or Goex 3f and the accuracy is exceptional. With 85 grain, that big chunk of lead has plenty of knock down power also.

No Excuses Concials can be found here. The guy is great to work with.

That is a great rifle your shooting. Give it some time it will come around...