I think that is why Jen didn't really want to tell us it was rubberband day.
Your howitzer sounds great, 300 ft.
It was six feet long and a bear to load. The range depended on the diameter of the tube used. I'd cut tubes into rubber bands. A 1 ft band (trailer tire tube) cut 2 inches wide would lob out to 300 ft with no wind. The barrel was a lawn timber with a notch cut on one end and slot with wooden dowell landyard operated trigger on the other end. A 2X4 carriage with lawnmower wheels and elevation was adjustable. I later modified it to be more like a slingshot with the tube secured at one end. Golf balls can be very destructive!