Carbine = short rifle, or something with a stock that fires a pistol caliber
Clip = Device to load a magazine (either fixed or detachable)
Magazine = what actually feeds cartridges into the firing chamber of a gun
A GSG-5 is NOT a machine gun, as it will only fire one round with a single pull of the trigger (it also isn't a knockoff of the mp5, it just looks the same)
Aux, I would argue that a MG does not necessarily denote the inability to fire single rounds. Group Therapy mode is needed, but I'm sure you wouldn't call the HK-21 *not* a machine gun, just because it has single shot capability.