That is the rear end of the barrel where the shell is inserted.
Near the breech.
From the breech.
From the breech
Where the shell is loaded.
Autos and pumps, on the receiver. Over/Unders are undr the breech locking lever
In front of the butt stock .
Less than $100.
Serial no. C8172
The Darne sliding breech shotgun comes to mind.
I'll take it for free :)
Try e-gunparts.com
Need more details.