Sorry, but the only bullet I have ever tried (and still use) in the Huntsman is a 400 grain PR Bullet Ultimate 1 hp. I have never slugged that rifle's bore either, so can't really say which size from Bull Shop you would need. It SHOULD be .502 as that was the H&R spec that I recall reading somewhere, but check anyway.

In .50x caliber, any of Dan's bullets of 500 grains or less will be western state legal (less than 2X).