Hello, everyone! I liked to thank you for viewing this instructable, this was in fact my first instructable and your support is really appreciated to me. This is going to be easy to build and it doesn't take up too much time to build it. (about 5-10 mins). Note that this should be built before the version 1.8. Ok, let's get started!
This TNT Cannon is able to fire a very large radius in a 360° (surround) mode. Let's start with the "You will need".

☆You will need:

•20 slabs (can be made in any material)

•4 buckets of water

•68 blocks that redstone can be put on (eg. dirt, cobblestone)

•50 redstone dust

•5 redstone repeaters

•1 button

•23 TNT (for one-time launch/activate)

Now we've got the items, let's get building!

Step 1: Building the Base

Start by make a 6x6 square out of the slabs (pic. above the other two), then fill the inside with water using your buckets of water from each corner (put them on the inner corner of the slabs). It should like like this when you insert the first water bucket (bottom left pic.), and like this at the end after you inserted all the water buckets (bottom right pic.). Now, let's move on to step 2!

Step 2: Building the TNT Slots

Now, you'll have to go up 5 blocks and remove the 4 blocks that is placed under the 5th block (Use blocks that redstone can be put on). Now that's done, you'll have to build a 8x8 floating ring from the 5th block. Next, make an inner ring of 4x4 on the same level where the 5th block is on, now put another ring of 4x4 on top of the previous 4x4 ring. Great! You've finished step 2, that means, to step 3!

Step 3: Redstoning

Next, put redstones on both rings (the 4x4 ring and the 8x8 ring).Put a blockabove the inner 4x4 ring and place a button on the side of the block. Then, make a platform for your redstone wiring. Now start placing your redstones and redstone repeaters like this picture shown above (Redstone repeaters may depend on your computer's speed). Now that you've finished this step, get ready for the last step of this awesome instructables!

Step 4: Placing the TNTs

At this last step, you'll have to place the TNTs in the inner ring and in the outer ring of the schematic. And you're done, you've finished building it.

BUT WAIT, before you even try to test it out by clicking on the button, you might have to backup the world so you can reload the world when something goes wrong.

Thanks again for coming to my instructables! Please share this if you think it's good and don't forget to follow me for more Minecraft (or other) builds!

You're welcome to my YouTube Channel:

https://www.youtube.com/user/COOLCREEPER69