The rear of the barrel IS the chamber.
A piston is the device inside the air chamber that compresses the air that is needed to propel the BB when it is fired. Every time you pull the trigger the piston forced air in the air chamber forward and the air fires the BB down the barrel.
The chamber is the cavity in the rear-most end of the barrel where a cartridge is inserted, prior to firing. The chamber walls are usually significantly thicker than those of the rest of the barrel, as the chamber must withstand the majority of the pressure generated upon discharge of a cartridge.
Barrel marking, chamber cast.
Chamber or barrel.
I just bought one at a gun show in fair condition for $70.
A magnet in the seat of the barrel
The original factory barrel will have the name Winchester and the chambering of the gun stamped into it.The color is another tell, the color on the chamber should match the barrel exactly.Look at the joint between the barrel and the chamber, can you see any bare metal at the end of the barrel?A replacement will have to have had the threaded end of the barrel milled after it was blued to get it to fit right.
It is usually marked on the chamber or barrel.
if the gauge and chamber length are the same
It depends on the model. Rifles usually have one behind the barrel or under the barrel. Pistols may have one under the barrel or in the grip.
Muzzle