"Ammo for the 5mm mag has been available for a couple of years now...but no new guns."
Yes, and if anything, that is a FAR more useful and reasonable scenario.
Putting the ammo back into production, but no new guns, at least ensures a supply of ammo for the guns that are in circulation, while new guns without a new supply of ammo... what the hell is the point in that?
Winchester, Remington, whomever, could make the most wonderful and useful gun ever seen by man, but if it's not chambered in a cartridge that's already at least marginally available, then what good is it other than for use as a club or a prybar?