Rotary bolt guns can open on firing if the contact between the bolt and the rib extension isn’t in the correct spot.
That’s exactly what’s wrong with my Sterlingworth Pin gun. Otherwise the gun is in very good shape. I just bought it a couple years ago from a reputable member and am very happy with it except the popping open issue. It was driving my local gunsmith nuts as he kept adding stronger and stronger lever springs which worked a little.
I finally talked to Jason at Pumpkin Mountain who explained to me how critical that rotary bolt-rib extension contact angle was.