USA: Stevens Model 520 trench gun with bayonet. Used in WW2 and up to the vietnam war
Germany: Walther semi auto shotgun. Holds 4 rounds, made in 1920s, competed unsuccessfully against the Browning auto-5
USSR: Mosin Obrez. Handmade modification for partisans who needed a concealable weapon. Goes into the sidearm slot. Not as accurate as a normal mosin but still hits hard.
Japan: Murata hunting shotgun, donated to the Japanese army by civilian hunters