try taping a spent hull with painter's blue tape, same problem brand, fire the bottom barrel, open and see if there are traces of primer being dragged. Re-cock gun, repeat procedure with top barrel. Issue could be failure of gun to rebound striker which isn't as noticeable on harder primers. Might be easier to see on unfired shell but with tape over primer or maybe just blackened with Sharpie. It goes without saying, but I will anyway. Don't try candle sooting on unfired primer. Gil