Starwarrior25 here! I'm going to give you guys a quick rundown of my TNT cannon I invented!

     This TNT cannon is based off of TNT Minecarts. The unique function of these minecarts is, when conditions allow, multiple minecarts will stack physically in the exact same place. This allowed me to create a cannon that can create an immense punch, and fire TNT further than before, while conserving space down to an extremely small area.

How it works:

     TNT minecarts are placed on the powered rail. They then fall down onto an activator rail, and, when more minecarts are placed upon the powered rail, stack on the activator rail. There is no limit to how many minecarts you would like to place. 
     Next, a single TNT block is placed at the end of the cannon between the two raised Andesite blocks. 
     Last, the button is pressed(By the way you might want to stand back as the cannon fires instantly), lighting the TNT and almost simotaneously retracting the piston, causing the minecarts, which causes them(somehow, it may be a glitch) to explode, sending the TNT flying!

Pros and Cons:

    This cannon is small and has the potential to pack an incredible amount of power into the space of one block, unlike older cannons which needed massive space to load a lot of tnt into one block.
     The cannon fires instantly, meaning the TNT can travel far without exploding in mid air.
     The cannon is very cheap, costing very little in resources.

     The cannon fires instantly, so there is almost no time for you to back up and escape the blast radius. However you can use a bow and arrow and use that to hit the button.
     The cannon destroys the activator rail, meaning a new one must be placed every time before loading it.
     The cannon requires a minimum of 5 TNT Minecarts loaded, else it will not fire(This is why I believe it to be a glitch). You can potentially use 4 but I do not recommend it as it doesnt always result in a clean explosion.
     

     Anyways that's basically it! Hope you guys enjoy it! A diamond or favorite or comment would be awesome :) Peace!