You should be shooting close to the same velocity in both loads ( 2680 vs. 2750 approx.) and I don't think the Whelen has enough velocity nor enough spin to compromise a common cup/core bullet let alone a solid copper, I would give the barrel a good cleaning (just to be sure) if that didn't help I would have Barnes evaluate the bullets, might try shooting into water at about 75 yards and see what it's doing.
RR