Let's Build a TNT Cannon to get back at those Creepers.
Step 1: Step 1: Base
To begin, you build three blocks across, two up and one back.
Step 2: Step 2: Legs
Step two is to add the legs, this has no effect though and is strictly cosmetic
Step 3: Step 3: Barrel Base
Step 3 is to add two iron blocks on the end of the base, what you built in step one, and in between them you place the bottom part of your barrel. Then on top of the two iron blocks you just placed, you add the upper parts of the barrel
Step 4: Step 4: Barrel
Step 4 is to add the full barrel, nine out
Step 5: Step 5: Firing Chamber
Step 5 is to add the "Firing Chamber" This is where you will keep your water and it adds a mounting point for another part of the cannon later. At the back of the barrel, place a block the same material as the rest of the barrel. You'll build two up and two out, then you will knock out the lower left hand block and the upper right hand block as I have.
Step 6: Step 6: Seating
Step six is to add the seat for the gunner, this allows the cannon to be maned by two players, counting the block at the end of the chamber, you will add two layers of base material, three out and three back, knocking out the middle block of the top layer, it is in this space you will place the mine cart and rail. There is only one more step before we move to the Cannon's Electronic parts.
Step 7: Step 7: "Bolt Asmembly"
Step Seven is to add the water to the firing chamber and let it flow as far as It can. at the end of the flow, you will place a stone slab, on the left, you will place another block the same material as the barrel and another stone slab on the left. The cannon will still fire without this slab, but it's accuracy will be reduced. This is the final step before we move to the Cannon's Electronic parts.
Step 8: Step 8: Trigger
Step Nine, the base of the cannon's firing circuit should look like this. Do not use levers for a trigger, there is a risk of reload discharge.
Step 9: Step 9: Charge Delay Switch
Step 10, The Charge Delay Switches should look like this. The Repeaters must be set to full delay if the cannon is to function properly.
Step 10: Step 10: Charge Igniter
Step 11, the Charge Igniter should have Redstone going from the Base Circuit all the way to the stone slab on the right side of the barrel.
Step 11: Step 11: Locked and Loaded
Step 11, Locked And Loaded
Step 12: Step 12: Boom