Rail allows you to attach accessories.
Size.
Mainly in the stock
Stock dimensions and sights
One barrel
A sub-machine gun fires pistol cartridges. A machine gun fires rifle rounds.
There is no difference in that either can be used for the other job. Of course there are hundreds of types of guns around in the building industry.
You will have to be more specific. Long gun or hand gun?
A person can buy a used Beretta 84 gun frame through websites like Gunbroker or at local gun shops. The frame is the main component of the gun where other parts are attached.
Stock and sights.
2mm in case length.
none.
Serial numbers are required by law to be on the frame (aka receiver) On the Beretta 92, they are located on the left side of the frame, about halfway between the trigger and muzzle. Other models may have them in different locations, but always on the frame. PS- by law, the frame IS the gun- everything else is a part that CAN be replaced, but the receiver IS the gun.