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.
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")
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")
Richard uses the Auto-5 to kill Manny Horvitz in his Atlantic City home on January 1 1923. After shooting Manny's driver with a weapon of smaller caliber, he waits on the front door and shoots him on his right eye as he opens, killing him instantly. Richard then hides the shotgun in his longcoat as he walks away from the house, while Horvitz's wife screams next to her husband's body. ("Resolution")
He unpacks his Auto-5 in a boarding house, along with the rest of his gun collection, after being kicked out of the Artemis Club by Gillian Darmody. ("Two Imposters")
Richard uses the Auto-5 as a backup weapon when he storms the Artemis Club on June 26 1923, the shotgun hanging from his belt. It is the third weapon he uses, after the sniping M1917 Enfield (his primary weapon) and a Smith & Wesson Triple Lock, and he uses it to kill his third victim, a Rosetti thug armed with a shotgun by shooting him on the gut. The body falls next to Gyp Rosetti, who is hiding behind the bar, and convinces him to run away with two of his men rather than trying to put up a fight. Richard makes more shots with the Smith & Wesson until he runs out of bullets, and when another thug who had been shot earlier on the ear with the Enfield appears behind him, he hits him in the face with the Auto-5's buttstock. Richard fires the Auto-5 a second time on the upper floor, hitting another thug on the side of the waist, but he fails to kill him and after struggling in the hallway he finishes him with a Colt 1903. ("Margate Sands")