Do you know of any single shot break action shotguns having steel receivers that would be good candidates for a barrel stubbing project? (other than the h&r ultra slug) Looking for something cheap to make into a 500 nitro. Are the CBC singles steel?

How about the Henry single shot? I've never shot one but it might work.

https://www.henryusa.com/shotgun/single-shot-shotgun/

What about buying a TC Encore frame and getting a 500 NE barrel for it? No need to wonder if the frame will hold up.

What about an old greener martini action shotgun. They are very strong actions.