"Slugging" is pushing a pell through the bore, feel for rough spots etc. Use a cleaning rod and look for clean rifling marks on the pellet.
Lapping is polishing the bore to remove any roughness, fire-lapping is fairly easy and can help improve accuracy. Hand lapping is more tedious. Both methods require polishing paste. I use fine valve lapping compound. You can google the procedure, or PM me.
A bad breach seal will cause inconsistency. This seals ur barrel to the compression chamber. Often it's a simple O ring, much easier to replace than the piston seal.
Have u cleaned the barrel? They often come filthy with oils/metal chips etc. right from the factory. The crown may be burred, as well.