The tiny cannon that you see above is fully functional. Pocket Artillery’s precisely machined mini cannon uses black powder to fire .177″ BBs up to 100 yards and are powerful enough to break glasses and shoot through coke cans… that kind of thing.
They are reasonably cheap, just be aware that the $29 asking price will only get you the cannon. You will have to acquire your own black powder and fuses. Needless to say they should be kept away from children and irresponsible adults.