It's a problem I find with NoExcuses conicals. The pure lead is so soft that even though he makes them oversize by the time they're half way down the bore they almost fall the rest of the way. The only way I can get them to work is to have the bore really fouled a lot.
Just the opposite problem of the Thor. The Thor is a good bullet but getting the proper fit is always a struggle.
I think the answer is to still fit the Thor to your bore like the regular Thors but have the slits on the base. Then you're not trying to fit bores from .500 to .503 with one hard conical. Make .500, .501, 502, and .503 with slits.