This TNT cannon is small, but effective. You can easily build these in TNT Wars and explode your opponent!
Step 1: The Blocks
Tip: You can use any block you like, but obsidian is the best because it is the most durable.
Place 3 blocks down side by side. Right across from that, add another 3 blocks. Add 2 blocks on top of the blocks at the right (there should be 1 block left with nothing on top of it on the right), and then add 1 more block on top of the 2 blocks on the right. Now one 3 by 1 structure should look like a staircase.
Step 2: The Dispensers
Add 3 dispensers like a staircase opposite the side of the blocks that look like a staircase. Then, add 1 dispenser on either end of the 3 blocks (the opposite side of the staircases). After that, put a dispenser 1 block back in the middle, then add 3 dispensers on top of the ones at the back.
Step 3: The Water and Redstone
First, add a slab at the front of the cannon. Then add water at the back. Second, put redstone on top of the staircase dispensers. Then add a lever on one of the dispensers. Third, put slabs in the corners of the 6 dispensers at the back, then add redstone on top of the top 3 dispensers of the 6 dispensers, put it on the slabs, and put it on the ground right behind the slabs. Lastly, put 1 redstone in between the redstone on the ground, and put a lever behind it.
Step 4: KABOOM!
Add tnt into your dispensers. Then, click the lever at the back. About a millisecond after that, click the other lever.
Then watch your cannon go BAM! KABOOM! and KABAM!