To properly fire an active shot (and avoid destroying your cannon), you need to be sure that your charge activates before the shot is activated.The charge (if you are just playing or testing) should land (TNT will fall when activated) and explode in water, otherwise the explosion will destroy nearby scenery and damage nearby mobs.The charge is an amount of TNT used to propel the shot.If raiding, make sure (unless you are using a hybrid TNT cannon) that the shot does not land in water.The shot is the ammunition it can be a player, a mob, a piece of TNT, arrows, sand/ gravel or anvils.TNT cannons operate on the principle that when TNT explodes in water it will not destroy blocks, but will still launch entities, including TNT that was already ignited, which is usually done with redstone (the charge is usually ignited immediately, then after a delay the shot is released).As of 1.8, it is also possible to build a TNT cannon that works with pistons and slime blocks. Note: these (as the whole article) talk mostly about the classical TNT cannons that work by shooting TNT by using other TNT blocks as charge.

9.6.1 Select fire, select range TNT cannon tutorial.

9.3.8 Human (or other entity) launching cannon.

9.3.7 One-button defensively ranged cannon.

9.3.3 'The Pulverizer' - An Ultra Rapid Automatic Slime Cannon.

4 Limitations, and ways to possibly get around them.

I test the same tnt launcher on a local world, a world of my own, and it didnt work either. The problem is that is not working on survival. The tnt launcher was the common design with dispensers around a 3 by 3 square full of water and a button to activate the 12 dispensers, and of course i was on the middle of the water square. I was playing on a friend's realm on creative, and i build a tnt player launcher, i tested it (i was still in creative) and it worked, it launched me very high, it was succesful but when i tried it in survival it didnt launched me, i tested it plenty of times because i taught it was like a 50/50 chance, but no, it didnt worked.

The player/mob should be launched high into the air. The player/mob is not launched high into the air.

Activate the dispensers and wait for the TNT to explode.Switch to Survival mode if you are standing in the water.Stand in the center of the water or spawn a mob in the center.

Minecraft current tnt launcher design