A short-barreled Browning Auto-5.
The Browning Auto-5 is a recoil-operated semi-automatic shotgun designed by John Browning and first produced in 1902. As in the Winchester Model 1912, there are long and short-barreled ("riot") versions, which are better respectively for offense and defense. The name references the fact that it can fire 5 projectiles before recharging.
History
Season 1
Al fires his Auto-5 during the Hammonton hijacking. ("Boardwalk Empire")
At least one of Davey Murdoch's men carries a Browning Auto-5 when they are unloading liquor for Arnold Rothstein in Atlantic City shortly after the beginning of Prohibition on January 1920, as does Al Capone when he hijacks the shipment near Hammonton, New Jersey. Capone hits Murdoch on the face with the Browning's butt when Murdoch asks him if he knows who he is stealing from and, scared by the noise that the deer make in the woods, fires on Rothstein's men killing Murdoch immediately. Murdoch's driver Simon escapes into the woods and Capone runs after him; he shoots Simon on the gut but (as it is later revealed) he fails to kill him. ("Boardwalk Empire", "The Ivory Tower")
Season 2
Richard Harrow oversees a liquor shipment's arrival. ("21")
Richard Harrow acquires an Auto-5 after moving to Atlantic City in the second half of 1920. He carries it while overseeing the unloading of liquor on the beach and its transport to Chalky White's warehouse in mid-February 1921, and also when Jimmy and him steal back the liquor from Chalky and re-sell it to Mickey Doyle. One of the men guarding Doyle's warehouse also carries an Auto-5. ("21")
When Nucky sets up Jimmy to get killed, Eli points an Auto-5 behind his back to make sure that he does not run away. ("To the Lost")