You may be able to give the barrel a good scrub with brush and cleaner. I know some barley twist barrels that have needed a good scrubbing after time. Some say they never need cleaning?!
It sounds like it your existing barrel is not how it should be. You could contact a gunsmith and ask them to machine either an LW barrel blank or a CZ barre blankl for your rifle. Do this if the scrubbing does not resolve it.
The american rifle and my Steyr are not pellet fussy really.
You can never shoot enough pellets