Are the barrels interchangeable by the customer on the handi rifles ?

Most require some fitting. H&R's tooling was not precise enough for interchangeable fit.

Also, ones originally made as shotguns or for revolver cartridges like .357 aren't suitable for high presure rifle cartridges. A little searching will find you the specifics on that. Since they're discontinued, getting barrels fitted may be more trouble than it's worth. The new Henrys look good in pictures, and are priced very reasonably, but aren't available as multi-barrel sets as far as I know. Maybe they'll offer that at some point.

Another option is to find a multi-barrel set for sale, but I'd do the research before buying, so as to know what to look for.