I once had a McGowen .50 barrel with those numbers (.500/.512 and 24" twist).

I put it on a rolling block rifle chambered for the .50 Alaskan and it was VERY accurate with RCBS 550gr. in 50/50 pure/WW alloy. I used both quite heavy smokeless and black powder loads, but nothing over 85gr. due to case capacity.

500gr. with 150gr. 3F? sounds really snappy/hot.