The M16 is insanely good on the PC version in headquarters, because you'll often find the defenders clumped around the area and a 3 round burst can kill 2 or even 3 people. And if you rapidly shoot you can clear a room with an M16 in seconds. Of course, that's also partly dependent on the accuracy of a mouse.
MP5 is nice too. I have a setup similar to Ben that I use on occasion. Usually in Headquarters. The other setup I like is the dual primaries of an MP5 silenced and an M16.








