You need to call Remington directly.
There is a .50 replacement barrel on Gunbroker.com
A replacement barrel for a J. Stevens Arms C20-A shotgun can be found at a local gun/firearms store. If the replacement barrel is not available on site, ask if the store can order it for you.
Try the gunpartscorp.com
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.
You can use the N-strike Recon CS-6 barrel (good gun, I recommend it)
From SAAB., Your nearest dealer can get a key to match your barrel.
find a replacement barrel and cut to the length you want
There is no restriction on barrel length but the overall length of the firearm must be 26.5 in or greater
Yes
Find a good gunsmith and machinist.
See related link.
I would start my search at Gun Parts Corp.they can be found on the web.