I just had a .50 B&M made up. Capacity is the same as the .458 win.
Sounds like an interesting rifle. What are you planning to hunt with it? How is the bullet selection?
Bullet length and depth of seating determines useable case capacity. In my Ruger No.1H, all bullets are seated 1/4" into the case, and that includes the long 500gr monolithics.
