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: