The 500 Linebaugh cartridges are all 510, and the 50-70 750 and 50 alaskan barrels JD Jones does are also 510. No problems there, but they may have gotten an exemption some time in the past...
I also remember JD making a 585 jdj blackpowder cartridge for the contender, but don't know for sure if it was in a handgun length.