I answered this question personally when Bullet stuffer emailed me, and hopefully we can work together to give you readers some better information after Bullet stuffer gets a chance to play with the information I gave him.
My most important recommendations were.
Use an LBT bullet that is made for your gun.
Don't use Unique (or similar fast pistol powder) For light loads go with one of the 223 class ball powders and work up to what you want or what gives top accuracy. For more power a ball powder recommended for jacketed bullets of similar weight, starting with the recommended starting load, and possibly being happy with that velocity level.
I've never loaded for or shot a 416, but there is no reason cast will not give near max power with decent accuracy.