Russian Loadout

Some weapons are faction dependent until you complete one service start with each class (this means that you've earned and unlocked everything for that class) Russian loadout weapons are restricted to the Russian Side and US weapons are restricted to the US side. To look at assault for example, you can only use the M16A3 and M9 while playing US and the AK74 and MP443 are restricted to the Russian side.  When you earn your first service start you can use any weapon, Russian or US while playing on either side.

Certain weapons are also always faction dependent even if you have a service star in that class.  The SMAW and Stinger are always only for US and the RPG and IGLA are always for the Russian side in the engineer class.

*EDIT** The M-9 and MP443 pistols are faction depended until you get 100 kills with each pistol, so when you get 100 kills with the M-9 you can use it on either side, in order to use the MP443 with either side you have to get 100 kills with that pistol even if you've already unlocked the M-9 for both factions (and vice versa).