This is a cannon that I designed in Minecraft that shoots quite far. I has a condensing charge. For those who don't know what that means, a condensing charge is a charge that fires before the firing charge and removes the spaces between blocks (as seen in pic 2) of tnt increasing the firing range. I hope that wasn't to confusing.
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Step 1: Building the Frame
The first step is to build the frame of the cannon. Make a shape like in pic 1 that is 15 x 3 wide. Then add 3 more layers on top of that with the middle hollowed out and the opening in the direction you want to shoot like in pic 2.