Minecraft wiki entry for TNT_cannons, associated text: When a TNT charge is primed, it will usually shift slightly to one side, which can reduce the accuracy of otherwise well-designed cannons. A simple solution exists: add guide blocks so that there is a wall on either side of the TNT when it is primed. This will prevent the TNT from shifting left or right, resulting in a straighter shot, with less diagonal variance. If your design does not allow for this, you could just make the guide blocks one block higher, instead of being on either side of the TNT. This works because TNT will jump up, then go sideways in midair. Note that in most basic cannons, guide blocks should be transparent if the TNT cannon automatically primes the shot-otherwise, the redstone that ignites the charge will power the block next to the shot, and igniting the shot as well. If this explanation is unclear, look at this diagram: