My son is working for a small studio now (this is the same son who was a tanker in Iraq) and mentioned in his last call about visiting a prop house that had a warehouse full of weapons and vehicles. I don't remember his exact words but historical authenticity and accuracy are not that high on the list of priorities. I suspect that actual, historically correct weapons are probably too rare and expensive to use.
Moviemakers have always used substitute weapons, mainly because of scarcity of the correct weapons. In Gunga Din, you probably noticed that the British were using Krag rifles. In Zulu, some Martinis were used but the men in the back had Lee-Enfields.
Sometimes they are lucky. In the pre-war production of The Four Feathers, which was set in, I think, the Sudan around 1900, they just pulled original uniforms out of storage in Egypt. On the other hand, they can sometimes have trouble with up to the minute stuff, although you wouldn't know it until twenty years later. In WWII movies made during the war, the Germans were often seen wearing the WWI style helmet instead of the later, correct version.
There are lots of historically accurate costumes that have never gone out of production since being introduced a hundred years ago, too, but costumes are easy. Charlton Heston's costume for some movie he was in was made by the same taylor who made the original character's unforms a hundred years earlier. He (Gordon, I think it was) and Heston were the same size. But judging from a lot of movies I've seen, it is next to impossible to get hair styles right.
