My gun shoots best when the pellet head is over 5.52mm. I had 15.9 jsbs thatt I had bought in May and they shoot excellent(I posted groups in June "Marauder shoots great") but when I ordered the same in June I couldn't hold a 6" group at 40 yards. The cphps are 5.53mm and shoot pretty good but not anything like the old jsbs.
As for the choke it starts about an inch and a half from the end of the barrel and seems to be OK.
I don't want to do anything else to the barrel and probably if I do it will end up trashed.
Thanks for the info, Jim