I believe that Hardcast may be slightly confused on bore diameters. Using a bullet diameter of .459" would be OK for a .45-70 or other .45 caliber centerfire rifle. But definitely not for a muzzleloader that has a nominal bore diameter of .451". Using pure lead cast bullets out of my .45 Hawken I've shot bullets with finished diameter of .449" up to .453". The bullets that shoot with the best accuracy are my .449" paper patched and the .451" and .452" are a toss up depending on how good I'm shooting that particular day. The .453" are much more difficult to load and show no improvement in accuracy with maybe even a decline.
But, pure lead or not, I could not see me even getting a .459" started in my bore. I don't think I'd even be able to get the base started.