The most commonly seen gun in the film is the older M3 "Grease Gun". It is not an M3A1 model because the guns clearly hav external charging handles on the right side while the M3A1 as divots in the bolt so the bolt can be pinched and pulled back. They also have a much higher rate of fire than the M3A1 model. Prison Military and Police (M&P) guards are seen using them, as well as practically every member of the Dirty Dozen, most notabley Major Reisman (Lee Marvin) during several scenes in basic training. In reality, the Grease Gun was never fielded in such great numbers but was likley the easiest gun for the film armorers to aquire.
While the M1 Garand was the most heavily fielded weapon of the U.S. Military during WWII, it is seen somewhat scarcely in the film, mainly in the hands of M&Ps and some of the members of the Dirty Dozen during the war games.
Several times the Browning M1919A4 is used in the film. First it is seen carried in a jeep by the blue team during the war games. Two are used during the assault on the mansion, one used by Roscoe Lever (Stuart Cooper) and Seth Sawyer (Colin Maitland) near the front entrance and the other manned by Tassos Bravos (Al Mancini) to combat the approaching German vehicles.
Several members of the Dirty Dozen keep Mk 2 hand grenades hanging on their belts, most notabley Robert Jefferson (Jim Brown) and Archer Maggott (Telly Savalas). While on the roof, Seth Sawyer (Colin Maitland) gets stuck after having a leg break through the roof on his way to disable the alarm tower. Immobilized, he throws two Mk 2 grenades at the tower to destroy it instead. When preparing to blow up the mansion, Major Reisman (Lee Marvin) and his men dump bags of Mk 2 grenades down the vents and then dump gasoline to make the building itself a bomb.
Joseph Wladislaw (Charles Bronson) is seen using a Beretta 21A Bobcat fitted with a sound suppressor to kill several guards at the gates of the mansion. While inside the mansion, he manages to kill a radio operator with it before leaving it there. Since the Bobcat pistol has a tip open barrel, this makes small acorn .22 blanks easy to load and unload with non-blank converted guns (which this clearly is since it doesn't cycle). This is likely why this gun was chosen since it was not made at the time of WWII.
German soldiers carry Karabiner 98k rifles during the final shootout at the mansion. Like many WWII films from 1960-1980, the MP40 submachine gun is inaccurately shown as the main weapon of the German army instead of the Kar98k.
At one point in the film, a German sniper is seen using a Karabiner 98k fitted with a scope (which appears to have infrared vision, which wasn't available on such a small scope at the time) and uses it to kill (???) with a shot between the eyes.
The first German sniper is hilariously seen using an American M1 Garand with the same fake scope mount, likely because of a lack of 7.92mm blanks or firing Kar98k rifles.
??? mans an MG42 to combat the approaching German vehicles. The gun is fitted with a 50 round drum. The same gun is seen mounted on a German vehicle but not clear enough to see.
Posey is manning the gun alongside Bravos. Strangely, movie doesn't show the fate of Posey (we do see Bravos dying), so I just assume he died also.
When Major Reisman (Lee Marvin) and Joseph Wladislaw (Charles Bronson) infiltrate the German mansion disguised as German officers, they keep Walther P38s as their sidearms and use them to shoot the guards outside the shelter entrance before sealing the people inside.
When the remaining members of the Dirty Dozen try to escape in an armored vehicle, the look back to see it is towning an unknown German Anti-Aircraft (AA) gun. Victor Franko (John Cassavetes) then jumps off and removes the hitch.
