Bullet sizing dies will not make bullets bigger, don't forget that.
.511” bullets in lead alloy will likely shoot all over the place in 50 BMG.
Follow the known basics and get your bullet the largest size that will chamber in your rifle. If you don't know where to start , get some chamber casting alloy and do a chamber casting to capture your throat area of the chamber. The bullet size that is big enough to fill the throat and contact the throat will be the most accurate for you with cast. Not so big that you cant hand chamber a round, but you should feel the bullet slide in the chamber throat while chambering a round with a cast bullet.
This will likely be .513- .515” --maybe even bigger if they will chamber. Check bullets from your NOE 850 gr FN mold with a dummy round for throat fit before considering any sizing at all. If they chamber--shoot them un-sized. If they don't chamber, use the smallest amount of sizing that will allow chambering.
You mention copper plating, the only thing copper plating does to cast bullets is give an amount of copper to copper your bore. If your cast bullets fit properly snug with a reasonable alloy for your load pressure, you will have no leading even with a simple tumble lube like Lee Liquid Alox or the new and better commercially available 45:45:10 @ $15/quart
.511” is a big mistake. That is the size for jacketed bullets in the BMG. Cast bullets that size will have major major gas jetting, heavy Leading, tumbling in flight and very poor accuracy.
Research the powders you may use. In general, for cast, the powder with the lowest pressure for the velocity is the best choice and gives a gentler soft start to the cast bullets with a pressure curve that is not sharply peaked. This generally means the slowest powder that will work reliably for your bullet weight.
Current Data from the Newest Lee 2nd Edition. The start loads for jacketed bullets will be about maximum for cast:
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