Diameter of the case is everything when it comes to calibers that the contender can handle, the .500 is way too wide to leave enough barrel and chamber steel to handle much if any pressure. Look at the 45-70 for example, and then remove another .05 inch from whats left.
I don't see the 500, or any other 500 being safe in contender, even the blackpowder barrels out there are only for 45 caliber, not 50s...even in low pressure black powder.
regards,
Graycg