Use one of the following entries with the "mpcheat changeclass" code. 

ak (AK47)
ak74su (AK74SU)
ar (SAW)
at4 (American rocket launcher)
b (Benelli m390 combat shotgun with 7 slug rounds)
f (Fists)
g (M16A2 with grenade launcher)
gp (AK47 with grenade launcher)
m4a1 (M4A1)
m4m (same as sf but with different grenades)
m9 (pistol)
mos (Mosin-Nagant sniper rifle)
r (Grenades)
rpg (Rocket propelled grenade)
rpk (OpFor machine gun)
s (M81 sniper)
s24 (M24 sniper)
spr (Silenced weapon with 4x zoom)
svd (Dragonov sniper rifle)
v (AK siris modified weapon)