The short answer is forget about it.
Having said that, my plt sgt, who has deployed to Afghanistan twice, and is headed back again, is taking his USP .45 with him. He is a senior MI NCO who will be working mostly with who we refer to as "OGA"s, "Other Governmental Agencies", in civilian clothes. Part of his job is to keep the enemy guessing as to who he is and who he works for. I talked to him a lot about it, he said, "When you get there, ask your commander what his policy is. He'll probably tell you no way in hell. If he says, 'out of sight, out of mind', then scrounge to see what people who are leaving are selling since they can't take it home with them."
Meaning, you can't take it in with you, you certainly can't take it home with you, and even if you score a good .45 from someone, you probably want to use FMJ ammo.
The truth is, if you join the army as a place to use guns a lot, or learn a lot about guns, you're going to be disappointed.