back to my suggestion re a legal form of gun that could be made by a model engineer so disposed.
if you look on the internet for ‘very pistol’ you’ll find very few entries. however the world opens up with information if you do a google search for ‘flare pistol’.
in my country flare pistols dont need to be licenced so they may be a valid topic for a model engineer to take on as a machining exercise. especially one in stainless steel.