armorer

(noun) An enlisted person in charge of maintenance and repair of the small arms of a military unit.

Synonyms: artificer

Usage: The soldier brought his gun to the armorer to have the sight adjusted.

If you click through to the full definition, the above definition is actually listed second out of three. The other two are as follows:

1. A manufacturer of weapons, especially firearms.

3. One that makes or repairs armor.

I can't say for sure how common this term is used in the military today, but to me, the first definition sounds the least antiquated.