Many of the soldiers in the film are armed with M1A1 Thompson submachine guns noted by their triangular "bunny ear" sight housing including Pvt. Kelly (Clint Eastwood) and Master Sgt. "Big" Joe (Telly Savalas). These guns were specially requested by Kelly for his special squad to find the gold behind enemy lines, which is why they are not armed mainly with M1 Garand rifles like a normal squad. Throughout the film, the M1A1 Thompsons sometimes switch with older M1 Thompson submachine guns with more complex bolts and cheaper bolted peep sights on the back with no protective housings.
M1 Thompson
The older M1 Thompson submachine guns are seen in the hands of many soldiers, which had more complex bolts and cheaper style peep sights which have no protective housings like the M1A1s. They are mainly seen switching with the M1A1s throughout the film as a continuity error.
M1 Carbine
Pvt. Fisher (Dick Balduzzi) uses an M1 Carbine which he usually keeps fitted with a rifle grenade launcher fixed on the barrel as his main weapon throughout the film.
M1 Garand
Despite the fact that the M1 Garand rifle was the most commonly employed weapon in the U.S. Army during WWII, the rifle is seldom seen and is never used or fired in the film.
FN BAR Type D
The Belgian FN BAR Type D light machine gun takes the place of the American Browning BAR for the film with noticable differences such as a larger wooden forend, a removable barrel, cooling barrel fins, a pistol grip, and a dust cover which were added to the gun by the Belgian army for it to meet their military requirments. It is most notabley seen in the hands of Pvt. Petuko (Perry Lopez).
I'm having a little trouble with the FN BAR users names so I'm leaving them blank. Anyone feel free to help me out here.