Im just a newbie in regards to muzzleloading but from from what I have read it seems maybe the primers you are using may be a hair long. If you really snap the breech closed does it co-k normally? If it does just get a shorter primer, check the firing pin bushing is screwed all the way in , you traded for rifle right maybe former owner didnt tighten all the way if he removed for cleaning? There are several posts with charts of primer sizes if you need to change just do a search.