“The first semiautomatic assault rifle was the Sturmgehwehr-44...” [MuttTheHoople, post 4]
Assault rifles are by definition selective fire, not semi-auto.
The original definition (which used appear in the DoD dictionary): individual weapon, magazine fed, selective fire, closed-bolt, chambers a cartridge of lesser power than the standard issue rifle.
The first small arm to fit that definition was the AVF-16 (aka Avtomat Fydorova), developed by Fedorov for the Czarist Russian military and adopted in 1916. It saw action in the subsequent Russian Civil War. Chambered 6.5x50SRmm Arisaka, the Imperial Japanese rifle cartridge, it generated far less kinetic energy than the 7.62x54Rmm M1891 Russian cartridge.
I am thinking the Stg. was the first one with a clip.