Without going into profile history, and assuming American, I'm guessing a land based service, either the Army or Marines. Seeing as the Marines had M16s longer than the Army did. I'm going with Marines.
That said, I have a hunch you might be European somehow. Maybe Germany from how you misspelled Mossberg? You'd have to be NATO, but I'm not super knowledgeable about the German services.
Underway? Joke? Definitely the Coast Guard then. That, or potentially the Navy, which would also make sense, but I'm going with the coast guard.