I just got a Lee .50 cal 250g REAL mold and some pure lead. After casting up a bunch, I measured the top band of the REAL bullet, it was 0.514". After driving it down the barrel of by 2005 Optima Pro (v1), I measured the groove diameter and it was 0.511" and the land diameter was 0.499". I took another bullet and whacked it to make it really fat to ensure full fit. After driving it down the barrel, I got the same measurements, 0.511" grooves and 0.499" lands.
I've shot a couple hundred Lee TL452-230-TC cast bullets in a Thompson Center sabot (the yellow one) and that loaded sabot measured 0.506". It required a firm push to get it down the muzzle but the combination shot really well out to 100 yards with the right load. Recovered sabots showed a strong rifling impression and there was never any plastic fouling in the muzzle even after a 30 shot session.