This bottle cap gun solves that problem of eliminating the cap.  Pop the cap then shoot it.  This is an original design based on the example seen on Thingiverse.

The left and right sides print nicely on a 200mm print bed but, they need to be printed separately to fit. You don’t need support.  I use 0.25mm layer height, 2 vertical shells, 4 top layers, 4 bottom layers, and 20% honeycomb infill.  When you compile the Gcode for one side of the gun you could include the Bottle_Cap_Gun_Insides.stl file if you want everything to be the same color. Or, print the “insides” file separately with another color.  The adhesion pads are for printing with ABS.  Trim them off after printing.

For assembly cut a 16mm and 32mm section of 1/8” (3mm) dowel.  The long piece is the trigger pivot and the short piece holds the rubber band in the striker.  Clean out the side-to-side hole through the striker with a 1/8” (3mm) drill.  Then push a doubled section of rubber band down through the hole (a 2.5mm hex driver works well for this).  Slide the short dowel through the hole catching the double loop (if you stretch the rubber band the dowel slides in easily).  Note: the photo showing the orientation of the rubber band in the striker.

Glue a 10x2mm magnet into the recess of the gun’s left-side-bottle-cap-track using super glue (CA).  This holds the bottle cap in place until you shoot it. (At one time I thought that placing a single magnet on one side of the track would give the bottle cap a spin for stability like a Frisbee but I don’t think there’s any effect.)  Drop a penny into the slot that catches the edge of the bottle cap (a US penny is 19mm in diameter).

Insert the 32mm length of dowel through the sear and slide it into the hole in one side of the gun. Place the trigger shoe in the pocket in the sear. 

Loop the two ends of the striker rubber band around the “thimble” and use M3x20 screws and M3 nuts to fasten the gun halves together.  Loosely fasten the screw at the bottom of the grip first to hold things in place.  It helps to stretch the rubber band to get it snapped into the pocket around the thimble (just cock the rear sight/striker to help slide the rubber band in place, un-cock it before fastening the muzzle screw so it lines up square with the nut).  I think the thimble protects the rubber band from cuts due to the threads on the muzzle screw.

Important Instructions for the Bottle Opener 

1. Cock the rear sight (pull it back).
2. Open the bottle.
3. Point Bottle Opener at a worthy Target.
4. Pull trigger.

Rules for a Sedate, Civilized Bottle Cap Battle

You must drink the bottle contents before shooting the bottle cap.