Whenever you pick up a SGK-5 sub machine gun and some mags in pristine or worn condition, you are unable to take out the mag that is already attached to the gun. The inventory menu indicates that you can change/attach the new mag but nothing happens. Also when you look at the weapon it appears that no mag is in the gun but it clearly indicates in the inventory that a magazin is attached. However, on the logon screen, the magazin appears attached.
As long this bug exists, you can't use the SGK-5.