1. (It has worked for me at the power switch box and the stg balcony) Have one guy kill them self on one side of the box and another stand about half way on the other side. The guy at the very end of the box pulls out a grenade. While hes doing this, wait 3 seconds after he pulled it out and then open the box. When he finally explodes, start reviving him and then when it gives you the option to trade items, release and immediately hold x again. You should start reviving with a gun and once hes alive again you will automatically switch guns.
2.(requires at least 6200 points+plus the gun you get price:5000 PAP, 1000 betties, 200 kar, ? your gun you upgrade)
Go to the PAP and upgrade your gun. When you do this then immediately start running to the kar. Buy the kar and run back to the PAP. Pull out your betty and when you first see it then trade your guns.(you can only get 3 guns with this)