You can get a custom sizing die from Lee for about $30 delivered. http://www.leeprecision.com/cgi/catalog/browse.cgi?1173395810.5340=/html/catalog/custom.html#CUSTOMLUBESIZE

Stillwell Tool & Die can make you one up but it will probably be higher priced. OR, you can hone out a Lee .501 like I did.... WOW! Takes a while... but it works.

Sizing in pure from .515 to .504 should be done in two steps. Get a .510 sizer in addition to your .504. Lube then size to .510, check to make sure the lube grooves are still full, then take it on down. Dan at BullShop does that with the 560 grain RCBS bullet. Looks a bit slick-sided but it shoots good. I think I know what mould you might be looking at - the LEE 515-500-F would probably work if those lube grooves are deep enough.

Plenty of moulds can be used to make good .451 conicals but the .50X are another story. I've tried to get Lee to make up some moulds for us and add them to their regular lineup - but no luck. Bug 'em.