I can't find it with the search function for some reason, but a few months back we had a discussion concerning possible ways to increase range and accuracy with golf balls in a mortar.
IIRC, we pretty much agreed that something has to be done to put a uniform backspin on the ball. My suggestion was to put two screws through the top of barrel near the muzzle, protruding just enough into the bore to grab the ball and stabilize it as it exits.
I nominate you to test this theory
Can anyone help with finding that thread?