The Godfather is the 1972 crime film that was an adaptation of the crime novel of the same name by Mario Puzo. The film follows the members of the Corleone crime family as a mafia war breaks out between Corleones and rival crime families. The cast features Marlon Brando, James Caan, and Al Pacino. The film, which was directed by Francis Ford Coppola and would win Academy Awards for Best Picture and Best Actor and would be followed by 1974's The Godfather Part II and 1990's The Godfather Part III.
The following weapons were used in The Godfather:
Smith & Wesson .357 Magnum
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. Since this point in the story takes place in December 1945 it would more than likely be a pre-war S&W 'Registered Magnum', though it could possibly be 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 gun in the film likewise has the front sight that doesn't run flush with the barrel (clearly shown in the second screencap below), indicating it is a pre-war Magnum.
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, however in the Godfather Saga made for TV the gun mentioned to use in the hit on Paulie is a .22. For Clemenza mentions the load to be used in the hit is a .22 soft point.
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 its compact size, relative accuracy despite only having a 2" barrel, and having 6 shots as opposed to 5 shots, as in other makes and models of snubnose .38s. 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. Peter Clemenza (Richard S. Castellano) gives Michael Corleone (Al Pacino) a Smith & Wesson Model 36 for his meeting with Virgil "the Turk" Sollozzo (Al Lettieri) and Capt. McClusky (Sterling Hayden).
Smith & Wesson Victory Model
US servicemen in Sicily carry M1928A1 Thompsons as they pass Michael and his bodyguards in jeeps.
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.
Bolt-Action Rifles (unknown)
Don Corleone's bodyguards at the house all carry rifles while patrolling the grounds.