Try a .490 or .495 with some .024 denim(old jeans)to start.Can lube with olive oil.You want a little snug but not to hammer to start.You can varry patch size from there.
The guys that have 56 cal often use 54 balls with a heavy patch.
It is a 50. You have to measure below the crown. If it is the Hawkins made by TC or CVA, they did not make a .52. Start with a 490 roundball and .15 patch. See how that grabs the rifling Should be tight but not so hard that you cannot push it down with reasonable force. Never hammer a ball. Sometimes guns prefer a 495 ball and .10 patch. Experiment and see what works for you.