A slide for a real handgun is not a controlled item. The lower frame of a handgun is the part that has the serial number on it, therefore it is the only piece of the gun that is controlled since it must be registered. Barrels are also uncontrolled too, unless of course they're prohibited length for handguns.
Interesting note: If you have a rifle
barrel shorter than 105mm it is not prohibited, because it is not for a handgun. Pretty useless ballistically, but still legal.