Double Rook Rifles are very rare and depending on make it could be quite valuable in its present un-altered state. Here in Britain they are much sought after. They have antique status here if original and hence fetch high prices. Check with Smallbore on this site and he may be in a good position to advise before you alter it.
I have a single .360 Rook Rifle which will take .38 Special brass and I just load for it using .38 Special cases and dies and get good accuracy over moderate distances although it does have a tendency to tumble bullets over longer range.
I bought one that had been converted by lining to .32 S&W Long that has similar ballistics to the original. That makes a good option if you insist on altering it but do remember that double rifles can be absolute pigs to regulate to shoot both barrels to the same point of impact. I would be loath to alter it. Lagopus.....
