Don,I did a little "gunsmithing" yesterday, I now have a working prototype 6 shot, semi-automatic, Colt 1908 rubber band gun.
The attached picture is a family shot of the working prototype and "most" of the reject pieces. Counting the versions that are in the gun, I made 2 versions of the trigger bar, 4 versions of the trigger, and 6 versions of the sear. Plus 2 versions of both the left and the right half of the frame which aren't shown in the attached picture. This was all to fix 2 problems, the shot-gunning problem that you already know about, and the problem of the trigger jamming.
The first modified version of the sear fixed the shot-gunning problem. From then on it was trying to determine why the trigger would jam, usually in the fully pulled position. It took me a long time to determine that the rubber band used hold the trigger bar up was getting pinched between the trigger bar and the sides of the frame. The trigger bar runs through a cavity in the frame, there's some clearance, but not too much since the cavity is used to keep the trigger bar and sear in approximately the same plane. I increased the depth of the cavity at the trigger end to allow room for the rubber band, while keeping the original depth at the sear end so that the sear and trigger bar stay lined up.
This mostly worked, the trigger no longer jammed in the fully pulled position, but it didn't ALWAYS return to it's normal position. This I eventually tracked down to the trigger catching in the surface of the frame.
The way that I print the frame halves is with the outside surface up, and the flat inner surface on the print bed. This gives me the best surface finish on the outside where it is most exposed. Unfortunately this also means that the first printed layer over all of the required cavities will be a "bridge". I don't care what 3D printer you've got, the bottom side of a bridge layer will be a corduroy surface at best. I finally figured out that the square leading edge of trigger was sometimes catching on this corduroy surface. I radiused the leading edges of the trigger where they contact the sidewalls, and problem solved.