If you were running max loads I'd suggest you back off a bit before changing bullets, but you said mild loads so I think you could substitute bullets with no problem. Whether of not they will shoot to the same point of impact is another question but I think it should be safe. I've never gotten great accuracy with Rainer bullets I've tried in .38 spl, .357 mag, .44 spl and mag or .45 ACP but at least they don't lead the bore and are cheap. I didn't know they were making .458" bullets.