Tom's correct 20/577 is an old cartridge. Ive been told there werent many guns made for it though. I gave the same idea lots of thought; but ran into a few roadblocks. First, since its a BP cartridge I wasnt particularly concerned with pressures as I would be regulating the powder. I figured if the gun was able to handle factory high brass loads it would take my lite starter loads, and we'd see from there. Sleeving a 16 or 12 gauge seemed the best option rather than trying to insert a barrel inside a 20 ga. Having custom dies made is never a problem if you dont have to worry about paychecks, for the rest of us it gets pricey. Then ofcourse theres the issue of brass, if you use Magtech shells, they're cheap and available; but they use rifle primers. Rocky Mountain Cartridge will turn you cases from solid stock at a higher price. Im not sure if they do necked cases though. My biggest concern was how I was going to regulate the SxS I wanted to use.
GOOD LUCK!
