A shorter case on any subsonic round would be best to keep load density up. The 50AE would fill this better than the 50b for that reason. As for the 458, shorter is better again. Here is the 458 Short with 350, 405, & 600g projectiles. I designed this around 9-10" brls. She will get a workout thursday time permiting
As for the 338, with the MGI bolt being released soon I am planing on running the 338BR case & just down load for the subsonic 338.