I have a FN action with a 20.5" 358 Norma that has some barrel rust so needs to be refinished. Action would be fine. Has a set of Williams peep sights.

Need a guess as to value as a barreled action or action alone.

An Interarms action alone just sold for $465 on Gunbroker.....Yes it was a .375 H&H length but yours could also be so modified.....Personally, I'd keep the barrel as the guy that wants the action probably don't want the barrel.

Its probably worth the price of the action. A 20" 358 Magnum isn't a big seller IMO..I pay $300 to $400 for late model FNs or Mark Xs..but don't use them much as I prefer Milsurp Mausers. 1908s, Chilean or Puruvians or G33-40 small rings..but that's just me, not knocking FNs..I do like the old 1935 FN Single C rings btw, but they are rare...

Yep I know the barrel would not add much if any value

I would put a price on it and list it on the gun selling blogs like Guns INt. Guns America and AR, Its a great caliber and some folks like short tubes in magnums, such as cowboys, guides etc. you never know..Try about $600 and see where it goes..you can always go down on a price butcha can't go up.