I believe the canno they use at the Encampment at Colonial Williamsberg is about what you have an mounted on a Grass Hopper.
You might see if they can help you.
I must admit I was wondering about shooting golf balls with a Gatling. They are small targets. I once hit one , in flight no less, with an M-60 GPMG. Talk about your slop shot, "No, I showed you once, you don't need to see it again" remain calm, un affected and walk away in total control. Not again in a million year would I count on that.
Now start another thread and tell us about your Gatling or I will be forced to write more-kBob Tales of Wonder.
-kBob