Not really a gun, but gun-adjacent. Early engines on aircraft (and other things) were often started using cartridges. These resembled a 4-gauge shotshell, but didn't contain any shot, just powder. When the pilot wanted to start the engine the cartridge would fire, driving a piston connected to a screw, which would rotate the engine for several revolutions. This was to avoid having to use a "pony engine', which worked fine on ground equipment but was too heavy for use on aircraft. This was later replaced by pneumatic and electric starters.