At the risk of having some devoted Casters not happy with me, my apologies, but with many good cast bullets available I can't see wasting the time casting. Now Casters, don't get me wrong Casting can be very fulfilling and part of the big picture for many of you, its just not for me. Yes, I have done it, but it has been many many years ago..........
Ok
@Get Off My Lawn answer me this, at SubSonic velocity exactly why do you want a such a heavy bullet? Either in .458 or .452? If you say penetration, if that is your answer, and you are talking deer/pigs and such, then you don't need any more penetration than what a 300 gr flat nose cast bullet can do at 1000 fps. A 300 gr FN Cast will blow through broadside, rear to front or severe angle of any deer or pig or even bear you might shoot at SubSonic. Since buffalo and elephant are probably off the list, then I am confused as to why you need heavier bullets?
Indeed if you are able to find the bolt gun of choice, in .452 caliber, honestly I believe a FN 300 will do anything you ask of it at SubSonic velocity.
Make life easy on yourself, and start with the 300s and see what you get.........
I was in Zimbabwe many years ago. I shot a really big Sable with 400 gr Swift A Frame in 458 Winchester. The animal dropped to the shot on the spot, but was still doing some wiggling when I walked up. In those days I was carrying a tiny little 5 shot Taurus in 45 Colt, it was loaded with standard loads of 250 Cast FN with 8 gr Unique. In the little Taurus 2 inch gun, velocity was 700 fps. I finished the Sable with a round direct frontal in the chest. Later digging the bullet out, it had traveled over 3 feet of Sable at 700 fps.
I don't know, maybe I am missing something else or another objective, but I would start with 300s at Subsonic and see what happens.........