The only projectiles I've fired with this particular cannon are 5/8" diameter wood dowels. I used cloth patches with the dowels but still left them loose enough to easily slip down the bore. I also fired a steel ball recovered from a broken PC mouse (rubber covering removed). It was about 0.625" in diameter. I didn't use a patch but raised the barrel elevation and was careful that the ball stayed right against the powder charge.
If you want to try lead round balls, check out Track of the Wolf:
http://www.trackofthewolf.com/They have .690" round balls, pure lead, hand cast, $8.00 per 25.
I intend to order some one of these days. However I have a lot of mouths to feed and rely mainly on cheap projectiles like concrete filled beer cans, old golf balls, dead C-cell batteries, etc:
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