i believe i read in an article that muzzle loader primers and shotgun primers were not the same as the ignition on muzzleloader primers are hotter. I have no way of proving that though. makes sense since the muzzleloader is not shooting smokeless powder in a standard case and there are variables on how tight each load is packed in the barrel. however I can also see the manufactures just putting standard shotshell 209 primers in a muzzleloader primer box and charging more for them.