I'm not sure what their problem with it would be. Other than someone screwing up and using the wrong shell. Maybe the bolt travel length is an issue. But they are sure saying DO NOT DO IT and they dont offer a 3.5 length rifled barrel. Oh well. I'd say go with your better idea anyway and get yourself a nice muzzleloader! I just got back into them myself a couple years ago as I am tired of the dang recoil from my slugger. Plus, with my TC anyway, I have more range than I do with my 12 gauge slug barrel. Lots of MLers out there nowadays that, with the right load and bullet, can be easily effective out to 250-300 yards. My barrel prints under 2 inches at 200 and can often get dang near one hole 5 shot groups at 100.