Ahoy, Gardenwargaming friends..
I just completed customizing the Big Bertha cannon and cannon ball.
You need two cannons to start with..cut off front section of one of the cannons, and glue it to the barrel of the other cannon....Split the original cannon ball with your handy Swiss Army knife,(or suitable tool)...install larger spring inside cannonball and reassemble...I used "super glue".
The modified cannon ball shoots about three times as far and hits quite hard. (Be careful around the kids!) The stronger spring will probably wear out the firing mechanism sooner due to the added friction on the latch.
Enjoy, and happy hunting!
Jimbo