If i wanted to use the .452 Lehigh 250gr in a bore that is very close to .500 but not smaller, i would try 3 sabots. I would try the MMP HPH-24, Harvester Short Black and the Harvester Black Crush Rib. I like 458s but in a tight bore the MMP Orange at .506 is very tight.

My guess is the Harvester Short Black should work very well but be a little on the tight side. My ULA 50cal has a real .500 bore and the HSB work very well with a .452 bullet. Ive heard the Accura 50s are very close to .500 also.