Hit the tilde [~] key, and type in

 mpcheat changeclass #

 to get the desired weapon, where # might be

 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

 Mark all courses as completed:

 Open the file /System/Save/yourname.txt Replace your line with this:

 511,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15 ,15,15,15,15,15,15,15,15,15,15,15,15,15,0,

 Note: May not work after submitting results to an online server. Always make a
 backup before overwriting files.