"The Parasite" - it attaches to the side of a larger object and can be rather unfriendly. This one is for airsoft use, and I keep one attached when going to CQB games. It is made to use PPS gas shells, so similar sized shells such as Madbull and Dytac should work as well.
Only four parts to be printed. The only other part(s) needed are 1) an appropriate spring for the firing pin button, but it will fit 4 ballpoint pen springs if needed; and 2) a pair of 6mm dia. x 2mm magnets to assist in keeping the barrel closed, although I haven't found these necessary in my games.
To assemble, rotate the body onto the mount and put the small tab in the slot. Slide the mount onto the picatinny rail where it will live. Insert the firing pin spring(s) & button and push in as you slide the rear plate in. This slides down over the button and locks the mount onto the rail. Done.
NOTE: Do NOT attempt to load real shotgun shells into this as it probably won't fire, and if it does it will likely cause serious personal injury or death. Don't even try it. Also, if you are in the U.S., it would probably be advisable to print the body in that horrid orange as required by law.
The mount should be printed with the large end on the build plate and with supports. The body and firing pin button should also be printed upright. I made mine in PLA and it works well.