Dowrat, you might get away with a change of patch material, without working your muzzles. I use 8 oz and 10 oz denim that I purchase by the bolt at a fabric store. They are .018" and .021" respectively, and hold up to shooting really well. I precut my patches though that shouldn't make any difference. Buy a yard of each and give it a try.
If you still have these issues, you will need to polish the muzzles in the manner that Daryl has described so many times here...abrasive cloth or paper pressed into the muzzle with the tip of the thumb and the rifle rotated back and forth. After a time, turn the rifle 180 degrees and do it again, so that the muzzle receives even wear all around. You will be polishing the sharp corners off the edges of the rifling as they exit the bore. Having done so, when you seat a ball on the muzzle, the stretching material will not cut but mold itself into the soft lead. Your loading will instantly become much easier and your accuracy will improve too. Many guys here have tried this and have reported a dramatic improvement in everything dealing with loading.