Multiplayer Cheats
first in the game you must open the consle by pressing "Tab" or press "~". Then you type in mpcheat for any multiplayer cheat.But if you want to change your gun you type in mpcheat changeclass "x". X meaning the gun you want.here are all of the different guns you can choose.

R=M16A2
AR=SAW
G=M16A2 (w/ Grenade Launcher)
S=M81 (Sniper)
AK=AK47
S24=M24 (Sniper)
F=Fists
M4A1=M4A1
AK74SU=AK74SU
GP=AK47 (w/ Grenade Launcher)
RPK=OpFor Machine Gun
SVD=Dragonov Sniper Rifle

Some other cheats are:
mpcheat ghost=no clipping
mpcheat god=god mode
mpcheat fly=you can fly

Dont use these cheats unless you are allowed in that server or it might cause you to get banned from that server.