I`ll go with the Bushmaster (ACR) ... I fired from it quite a few times , beautiful gun , i must say Oh and For Your guys Information ACR Stands for "Adaptive Combat Rifle"
I like the M16 assault rifle. Also the magnum sniper rifle is very powerful. In counter strike(the one on the normal xbox) I would always choose it then find a spot to hide out at and just snipe other people. It only took one shot to kill people it they didn't have any body armor. It takes two shots if they have body armor though.
Anyway, My favorite Guns are M4a1, Scar-H, TAR-21, .44 Magnum, and AA-12. They are all Kick *** ) By the way, all my favorite guns came from COD:MW 2. ahah.. )
I was watching this one video on youtube (no, I don't remember the name of it), and they had this thing where you could mount two of the AA12's and it was basically a sentry gun.