I had one old J Stevens 325 double fire upon closing once. I never figured out the cause for sure & could never get it to do it again. I really treated it pretty rough (Unloaded) to see if I could get a repeat & it never dropped the hammers again. It was timed to the point you could load it without it cocking, so all I ever came up with was that the sear didn't fully engage, but just caught enough to hold till it closed. The gun wasn't slammed shut either, just closed normally. After that I always made certain I opened it far enough to be certain of full sear engagement & it never occured again.
I had the exact same thing happen. I just got lazy in opening the gun to reload and didn't open it fully. I had shot a single bird and was only reloading one barrel, so there was already less resistance to opening and the "feel" was different/lighter than after a double-volley. In any case, I opened it just enough for the sear to barely engage, and the closing of the gun was just enough to dislodge it from the bent. Fortunately, I had stuck to my standard practice of pointing the gun in a safe direction to close it. That was the day I really understood why I had done it that way for all these years, but it was also the day I learned to be more deliberate when opening a fired gun. The same gun has had cases of shells through it since without a repeat.
