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.
The frame.
A homophone for a large frame could be "a full frame."
It is the frame of the gun. With present day guns, the part with the serial number on it.
It is not a large frame.
The receiver is the frame of the gun. When a gun is marked with a serial number, it is stamped on the receiver. It will be a different part depending on the type of gun.
The serial number is on the frame.
You will have to specifiy if it is a handgun, rifle or shotgun.
On the frame and or barrel
Gun shop, gun show, want ad
Normally on the frame/barrel/receiver
It is in different places depending on the gun, but typically, it will be somewhere on the barrel or on the lower frame of the gun.