Sounds obvious, but is the barrel seated properly in the action and is the retaining stud done up tight? 6" groups at 15 yards means that something is flapping in the breeze! I had a BSA Firebird that wouldn't group, just a tiny bit of re-seating of the barrel and it was spot-on. The pellets were being shaved by the loading block.

Have a look at the crown with a magnifying glass and see if you can spot any damage. But you would have noticed if you had damaged it surely? Try shooting it without the silencer just to make sure it isnt still a problem.

Other than that, is the barrel clean? Has it got some gunk stuck in it?

all the best with it.