only fly in the butter is that the H&R in their present configuration would be on the Unapproved list. Changing the barrel to octagon or the caliber would not change that. The action is the problem, and changing out everything else would not make it fly.
However, if you are just wanting something to shoot, I happen to have a H&R which has been rebarreled to 40-65 WCF with a 28 or 30 inch barrel.