Luca Brasi (Lenny Montana) checks a Smith & Wesson Model 27 Snub Nose before going to his meeting with Sollozzo and the Tattaglias. The frame appears to be that of the larger S&W N Frame. Also since this point in the story takes place in December 1945 it would more than likely be a pre-war S&W Registered Magnum. Or possibly a S&W .357 Magnum since a few were made between the ending of the Registered Magnum model and the start of WWII. During WWII, S&W only produced the M&P/Victory model. S&W did not number its models until the middle of the 1950s.
It can be confirmed that it is a Pre-war Smith & Wesson, possibly a Registered Magnum. All pre-Model 27 Smith & Wesson 357 Magnums had front sights that did not run flush with the end of the barrel. The one pictured doesn't run flush with the barrel, therefore it's a Pre-war Magnum.
Beretta M1934
One of the hitmen who shoots at Don Vito Corleone (Marlon Brando) carries a Beretta M1934. This also may be the weapon used in the hit on Paulie.
Colt Detective Special
This .38 snubnose Colt Detective Special makes several appearances throughout the movie, as the revolver truly was popular amongst Mafioso of the time due to it's compact size, reliability and accuracy despite only having a 2" barrel. One of the hitmen who shoots at Don Vito Corleone carries a Detective Special.
Smith & Wesson Model 36
Fredo Corleone (John Cazale) draws and immediately drops a nickel Smith & Wesson Model 36 after his father Vito is shot on the street. Sonny Corleone (James Caan) is briefly shown checking one and sticking it into his waistband when he sees Clemenza at the door.
Willi Cicci (Joe Spinell) uses a M1911A1 to murder Don Otilleo Cuneo (Rudy Bond) in a revolving door.
Submachine guns
Thompson M1928
Barzini's hitmen use Thompson M1928s with drum magazines when gunning down Sonny Corleone (James Caan) at the toll booth.
M1928A1 Thompson
US servicemen in Sicily carry M1928A1 Thompsons as they pass Michael and his bodyguards in jeeps.
Madsen M50
Rocco Lampone (Tom Rosqui) and another Corleone hitman use Danish 9 mm Madsen M50 submachine guns to murder Don Philip Tattaglia (Victor Rendina) and a woman he was sleeping with.
Shotguns
Unknown Double Barreled Shotgun
Michael's Sicilian bodyguards, Fabrizio and Calo, carry 12 Gauge Double Barreled Shotguns with exposed hammers. While in the novel the two use "Lupara" sawed-off shotguns, the movie instead shows their weapons with full barrels and stocks. Clemenza uses a hammerless model when assassinating Stracci in an elevator.
Winchester Model 1912
When Don Corleone leaves the hospital, NYPD officers are armed with what appear to be Winchester Model 1912 shotguns as they stand guard.
Rifle
Bolt-Action Rifles (unknown)
Don Corleone's bodyguards at the house all carry rifles while patrolling the grounds.