[kozak6]

It's not worth the effort. And unless you have an FFL, you are wading into very dangerous waters.

what would the danger be in case hardening the receiver of an antique rifle that takes a relatively hard kicking round?

It might blow the hell up. That's pretty dangerous, as I understand it.

well the case hardening is for decoration(like old colt navy revolvers, you can use chemicals which oxidize the surface of a gun much the same way parkerizing works

No, it isn't. Case hardening is nothing like parkerizing. It's a nearly obsolete surface hardening process involving extreme temperatures and quenching. Parkerizing is a cheap rust resistant finish obtained by boiling a part in a pot. Due to the high temperatures, you would be trading the original heat treatment for a surface process that would leave a soft interior. I'm not a metallurgist or a gunsmith, but there's the possibility that it may not even be possible to do it safely.

The simulated case hardening you have in mind is a different thing entirely. It wears poorly and tends to be widely derided on the few commercial firearms that have made use of it. It's a thing to avoid, not seek out.

basically my way of thinking was to take a gun that is VERY commonplace and make it unique so than in 30 years when this is the one antique gun that is still worth next to nothing, these might actually be sought after.

Nope. There's millions of similar "unique" Springfields, Mausers, Enfields, and such out there. It turns out that almost every single time, sporterizing them completely ruins the value. It's the unaltered rifles that are worth the bucks since there are so few of them left.

is probably going to die in the hairbrained idea phase of developement

Good.

Also, please note that not all Mosins are nearly disposable junkers. Many of them are rare and collectible rifles. I've seen chopped up Châtellerault Mosins. I've seen chopped up M39's. Always check the markings before you permanently alter a milsurp rifle. Although it doesn't happen often, sometimes you can strike gold. And if you do, then you can flip it to a collector for a pile of cash and then buy the rifle you really want.