Just get a 308. Can use it for anything and ammo is cheap and available. I just set up my Encore pistol for the 308 and a Burris posi-lock 2-7 scope. Sold everything else as I can reload the 308 with 90 grain all the way up 220 grain bullets. Match the load for the critter.
Check the price tag on 460 and 500 ammo, unless you reload, it's outrageous IMO. Plus I'll out shoot you with the 308 past 150 yards easy.
You don't "need" a pistol barrel to register it as a pistol. Just make sure you state that it's a pistol and do a WI handgun check. WHEN YOU FIRST PICK IT UP FROM THE DEALER.. That way you have proof the serial number on the receiver was background checked as a handgun.