I'm also a veteran, martinchaen, as well as a firearm owner. Just to make sure you understand you're not the only one with relevant first-hand real world experience. I know the importance of firearms maintenance and I do think it goes along hand-in-hand with learning to shoot. I really haven't met many civilian firearm owners who also don't care for their firearms after firing them at the range. According to the book, you can use active skills as substitutes for knowledge skills, so the skill appears to contain more than just the capability to perform the basic task but covers a broad swath of information.
When I field stripped an M16A2 to clean it, I tend to think of that as falling under the Firearms skill as basic maintenance. When the company armorer had to replace the hammer spring on my rifle after I noticed the firing pin leaving indents in the primer pocket without the round actually going downrange, I consider that the Armorer skill.
That's just me though and, like I said, I think the rifle in question should be changed using an Armorer skill check.