I tried to use a TNT cannon, using many different designs (almost every design I know, about 10) and all of them failed to work. The TNT in the water still created an explosion causing the cannon to explode. Try as many designs as you can, and please fix this soon, as many maps depend on the use of these cannons.
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Seems to be somewhat related to the items glitching through other blocks. Using a common design for a tnt cannon, a water channel with a row of dispensers on one side. I was able to make a working tnt cannon by adding a row of glass blocks above the dispensers and on top of the row across from them. Any blocks above the water channel could cause the tnt to glitch upwards. I also added an extra block at end of the water channel and let the water flow out the end of the cannon. Flight paths of the tnt are very random though.
if you want to add something in the water to hold the tnt up, use a fence and leave a one block gap to the tnt charge or one could glitch upwards on the fence and destroy the cannon.
Duplicate of MC-48573 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.