Hello, Morerams:
Nice to see Herbert.
With your GROOVE being .512", most would suggest a sized bullet OD of .001" - .002" over that in 20 or 30 to 1 alloy.
My Spencer 56-50 experience favors 20 parts lead to 1 part tin, both truly PURE, that CASTS .001" larger than the desired SIZED OD of .513" - .514"
I like a final sized OD of .001" - .0015" over groove diameter, which means your 20:1, AS-cast before sizing OD should be .512" +.0015" + another .001" (for sizing cleanup) = .5145" (as cast od), - .001" sizing = .5135". Whew!
Now, to get THAT .5145" as-cast diameter that cleans-up .001" to .5135", the mould has to be LARGER than .5145" to account for linear shrinkage. A .500" bullet in 20:1 alloy, can have linear shrink of another .0015" - .002", per metallurgical tables, but I find that if I specify "the mold should cast Lyman (new) #2 alloy to an as-cast od of .514" .5145", custom mould places like Accurate will know the shrink compensation and bore the cavity TO CAST TO your desired AS-CAST DIAMETER of .514" - .515".
You do not want your mold to "drop" castings that are too small.
If you ask most mould makers what their blocks cast TO in Lyman (new) #2 alloy, you might hear mush.
But Accurate has been so close and with no shift in the parting line, that my bullets have been right-on.
Then your bullet mould will cast bullets that can be sized to the proper diameter.
Herbert's near you. Maybe he can supply some sample OD bullets to try or
he may know a readily available mould that will be ok.
All the best,
El Supremo/Kevin Tinny