During the attempted assassination of Leo O'Bannion, his bodyguard's CZ-27 is seen on a table after he is silently murdered by Caspar's thugs.
M1928 Thompson
After Leo's bodyguard is killed, two of Caspar's thugs (Monte Starr and Don Picard) each get a Thompson M1928 submachine gun and attempt to murder Leo in his upstairs bedroom. Leo (Albert Finney) kills one with his New Service and takes his Thompson, fighting off the rest of the assassins. According to information and calculations from the IMDb "Goofs" page for this film, around 524 shots are fired from the 100-round drum in the Thompson: a 5-second burst from the gangster (28 shots), then subsequent 20 second bursts from Leo after taking it, each about 233 shots each. Leo's thugs arm themselves with Thompsons, as do police officers in the film.
Colt New Service
Leo O'Bannion (Albert Finney) fights off his would-be assassins with a Colt New Service he keeps on his bedside table. Tom Reagan (Gabriel Byrne) keeps his own New Service on his bedside table, using it to chase down Bernie Bernbaum briefly.
Winchester Model 1897
Some of Leo's thugs and several policemen are seen carrying Winchester Model 1897 shotguns.
Astra 400
Tic-Tac (Al Mancini) gives Tom Reagan (Gabriel Byrne) an Astra 400 to kill Bernie Bernbaum at Miller's Crossing. Tom fires it twice.
Eddie Dane, aka "The Dane" (J.E. Freeman) carries a Webley Mk VI chambered in .455 Webley throughout the film, using it to kill a thug at Verna's and to threaten her and Tom. A policeman at the Sons of Erin club scene is also seen firing a Webley.
Colt M1911
Bernie Bernbaum (John Turturro) keeps an Army-issued original Colt M1911 as his personal sidearm, using it to kill Johnny Caspar as he ascends the stairs at the Barton Arms. It can be identified as Army-issue from the lanyard loop and writing on the slide and as an M1911 from the grips and a straight mainspring housing. A man during the Sons of Erin battle also carries a 1911.
Smith & Wesson Model 10
One of Leo's thugs (Charles Gunning) who is wounded at Verna's by The Dane reaches for his Smith & Wesson Model 10 but The Dane keeps it from him. Sam Raimi, during his cameo as a nameless gangster, fires two long-barreled Model 10s.
Walther PPK
One of Caspar's thugs fires a Walther PPK at the Sons of Erin club.
Browning M1917
The police unveil and fire a Browning M1917 machine gun at the Sons of Erin club gunfight.