i just found a dope trick a few minutes ago..
i got the custom sniper rifle the first time by getting it on the LAST level and having it in my inventory when i beat the game, well right after that i went to do the hidden valley mission, and a sniper started shooting at me so i shot at him with my new custom sniper rifle while he was taking shots at me with his - anyway i killed him first and his rifle falls out the window and slides down the slope onto the ground!! i tried it again and it worked.
i'm not sure where i hit him, so don't bother asking.
but appearantly if you shot them while they are shooting at you their rifles fall out the window... plus maybe the m4 can be got this way???
ps. i tried this at the first sniper tower on the hidden valley misison.