At HellsMouth the dead troops left 45-70 Govt cases and the bad guys are carrying Trapdoor Springfield's that use 45-70s. Both were introduced in 1873. 8 years after the movie's events.
The move takes place during the US civil war which ended in 1865, There is a scene where a bag of dynamite is found, when found it was called a bag of TNT and while TNT was discovered in 1863 it wasn't discovered to be explosive until 1891,