Try a little automotive valve grinding compound on a small, round head, BRASS screw chucked up in a drill. apply a little pressure with the round head screw using the valve grinding compound to chamfer the leading edge of the rear of the barrel. This may take care of the issue. Also, you can purchase some brands of pellets with varying head diameters. H&N Field Target Trophy is one brand which comes to mind. You may find that going to a smaller diameter pellet is all that needs to be done.
Good luck!