Mine was .4565 as well. Bit tight - OK with jacketed .458" bullets as well.

I managed to pick up the 495gr. Schmitzer(spitzer) Lyman mould was well as the #457125 525gr.RN, both for $20.00, due to both being slightly undersized. They cast .4575" in my alloy and my rifle loves them, smokeless or black powder.

Those 18" twist GM barrels shoot!