The .308 vs .311 thing has always been an issue with US made stuff. I think the Ruger Mini-30 had the same issues.
That makes sense about the shotgun receivers not being as hard, sinc shotguns are usually lower pressure - but it still takes the same amount of machining, steel, parts, etc to make it. Hardening isn't that expensive of a process.
A rifle like this is the kind of thing I would buy if I happened to come across it at the right price, not something I'd go shopping for.