The improvised zip gun built and used by 41 year old army veteran Tetsuya Yamagami to assassinate former the former Japanese Prime Minister appears to be muzzle-loaded and electronically initiated. It appears to consist of two threaded gas pipes with end caps drilled to enable insertion of a wire which may be connected to a broken bulb or improvised electric match filled with black powder or match-head shavings to initiate a main propellant charge. Loaded in front of the charge would typically be paper wadding followed by ball bearings or other projectiles. A similar improvised muzzle loading gun was used to kill a police officer in South Korea in 2016.
Edit: More improvised firearms have apparently been found at the suspect’s home:
Confiscated from a 23 year old women in Argentina.
Slam fire pipe shotguns in 12 gauge and 28 gauge used by Argentinian street gangs.
Courtesy of Victoria Police.
Rotating barrel selection and in .22lr.
Things police find creeping around in Moldova.
The tape grip is probably the least eye hurting thing about it. Points can be added for having a decent amount of shiny new ammunition.
The bane of the New South Wales Police service.
Pour gunpowder down barrel, pour in steel bbs, light fuse, point at target and wait.