[NOTE:] This was originally published on my Shapeways store, tested using their printers. Tolerances may need to be adjusted.
A two-piece mortar cannon for PotP Outback designed for the cannon to pivot up and down in vehicle mode. The peg is 5mm with a 3mm peg protruding from the bottom of that. This allows you to mount it either on the spare tire ala the G1 toy, or on the roof if you leave the tire shield off. Both of these pegs are short enough to fit completely inside the 5mm port on Outback or Brawn's forearms, and can be handheld if you keep the elbow straight or tuck the butt of the cannon under the arm.