Resource is a step file, as it is likely that anyone using the file will need to tweak a few dimensions to match the capabilities of their printer. Currently I print them in ABS with ZERO horizontal expansion and the turret upright with support. This results in reduced from model size passages for BB's. I have also printed them in 3D850 and 3D870 PLA. both work
Ran well last summer, incorporated changes based on feedback/testing. prototypes bench test good, but need battle experience
step file contains all parts OTHER than piston/spring/magnet. sleeve insert in cannon breech allows tweaking internal geometry on simple part to match other springs and pistons. Piston MUST be ~0.6 inches overall length to function properly. Magnets are used to hold bb's in breech. fill tube MUST have ramrod to force BB's into magazine.
Don't forget to seal up piston port plug once set at correct location. I use JB weld on outside.
In case of burst firing, typical fault is binding piston or the piston port plug being backed off too far.
Print at your own risk, if you do not have good layer adhesion, fix that first before printing. Typical settings are 0.38-0.4mm line width, 4 layers on wall,floor, ceiling, 75% infil, cubic, print inner walls before outer walls.