A golfball is 1.680 inches. A lead ball cast to THAT size is roughly 1 pound.

I dont recommend shooting actual golf balls. They BOUNCE. Trust me, shooting a golf ball from a cannon is "exciting" unless you are sure they will hit SOFT dirt.

Heres my 1 pounder. Its modeled after the Soviet B-11 Recoilless Rifle but scaled to a golfball. I used a 1 lb round fishing weight mold for balls.
https://www.youtube.com/watch?v=s2Ar-2wx5bo

Im all about a 8 Bore and 4 Bore rifle but even I wonder about a golfball size rifle. It seems to me if you want more then you want something like THIS...
http://www.veteranarms.com/ReproductionMuzzleloadersandFlintlocks/Hand-Mortar.html

Firing (NOT ME) https://www.youtube.com/watch?v=Siepa2bjkg8