M16A2.
I simply love this gun. Why? It's like a permanent M4 with extra bullets. Why not? It's got slightly more power from reviews I have heard of, it has accuracy like the M4, recoil like the M4, etc. You can say it's a permanent M4-Adv without a silencer.
Disadvantages can result in that M4 Silver vs. M16A2 conflict. M4 Silver has a silencer, and basically same stats(although somewhat of a slight, wider spray pattern) as the M16A2. M16A2 is basically an M4 with extra bullets, and it's permanent and cheaper in terms of BP. Disadvantages of the M16A2 in particular - Nothing really. I like it and it's a good gun. It's for BP, though. If you want to add that on the disadvantages list.