I'd be very careful of making a .500 Nitro on a shotgun. We're talking chamber pressure here and while a double shotgun is slightly overbuilt for the typical 11,000PSI of a pelet load they are not designed for 35,000PSI.
; I know conversions have been made and so far seem to stand up to 2,000fps wiht 500gr. bullets, BUT, I really don't think I'd shoot one, myself. Black Powder, or equivalent loads yes, but not in smokeless. BP loads will run about 25,000SPI. I would have nothing to do with ANOTHER 3-1/4" .50 cal case. One's enough. WAYYYYY too much room in that case for black powder and especially smokeless. Of course, that's just my opinion.