Hogan (Clint Eastwood) carries a Single Action Army 5 1/2" Artillery as his sidearm in the film, as do several other characters, including the three robbers at the beginning of the film. This is historically incorrect since the Colt Single Action Army was introduced in 1873 and the French had withdrawn from Mexico in 1866.
Colt New Service
Colonel Beltran (Manuel Fábregas) is probably carrying a Colt New Service with stag horn grips as his sidearm, despite how anachronistic it is to the time period.
Webley Revolver
When Hogan (Clint Eastwood) kills General LeClaire (Alberto Morin), he throws an unknown model of Webley revolver on the table.
Rifles
Winchester 1873 Saddle Ring Carbine
Hogan (Clint Eastwood) carries a couple Winchester 1873 rifles on his pack horse's rig, but mainly only uses the one on his saddle bag. He most notabley uses it with the help of Sara (Shirley MacLaine) to shoot a pack of dynamite positioned to destroy a train track bridge soon to be crossed by a French munitions train. This is historically inaccurate since the French occupation of Mexico ended in 1866.
Some French soldiers use Gras 1874 rifles. Mainly when defending the fort from the Mexican Guerillas at the end of the film.
Springfield 1903
A French soldier is briefly seen using a Springfield 1903 before his arm is cut off by a Mexican Guerilla.
Remington Rolling Block
Mexican Guerillas are seen using Remington Rolling Block carbines during the assault on the French fort. Hogan (Clint Eastwood) briefly uses one to kill a French soldier.
Machine Guns
Gatling Gun
The French soldiers attempt to defend the fort by using a brass plated Gatling Gun of unknown make, before Hogan kills the gunners with a stick of dynamite. Hogan later uses the weapon to kill a group of soldiers, because apparently the dynamite killed the gunners but left the gun unharmed.