With both my 54 and my 50 GPRs it depends on whether the balls are home cast (Lee molds) or storebought (Hornady) ".490" balls. You're only asking about 50, so I'll stick to that.
With storebought my 50 likes either .015 prelube or .018 Walmart ticking with bore butter. It's pretty tight with the ticking, and almost right with the .015 prelube (very slightly too loose for my taste) and way loose with .010 prelube (holes in recovered patches).
With the home cast the ticking is awfully tight and just right with the .015- again to my taste- and blows .010 prelubes.
Not surprisingly, the Hornadys mic and average of around .491, while my homecast run .4925 to .4935.