Xbox 360, PlayStation 3, PlayStation 4
Re: PS4 Update 1.16 and PS3
Just prior to the update, I had built a simple TNT cannon that worked reliably well. Two 3 x 8 rows of dispensers facing each other and dispensing into a central trough with water. I had fired it several times without any problems or need for repairs. The best shots were traveling a distance of 100 blocks.
However, the first time I fired the cannon after the update, the whole thing blew up - literally. All the containers were destroyed. I rebuilt it several times, starting with small loads, and before I was even 1/3 of the way to filling all the containers, the same thing happened. I tried using obsidian and raising the dispensers and raising the water level. But even with a 3 block drop to an obsidian trough with water, the containers were destroyed. (Further testing indicates that 6 blocks high is a safe distance.)
It seems as if the water is no longer damping the explosions.
This initially happened in PS4 survival mode in the pre-update tutorial world. However, I have tried a new tutorial in creative mode and the same thing is happening.
Also appears to apply to PS3 creative mode. Simple trough with water and more than 3 TNT will blow up the container every other time. (Perhaps the first blast just weakens the blocks.) 6 TNT will consistently blow up the container.
RECAP (for PS4):
The behavior of TNT has definitely changed with the last update. As indicated above, you can no longer safely use cannons with a single water block in the back. Instead, the TNT needs to be totally immersed in the water.
I have tried out many different dispenser-fed cannons and have gotten them to work by making sure to fill the water in the trough where the explosions take place is "still water" with a depth that is at least equal to the height of the dispensers. A water depth of 3 seems sufficient for all cannons. But just to be safe, you can make the water as high as the dispensers. Also be sure to cover the trough so that TNT does not bounce out of the water.
This also works with "compression" cannons which use an initial "primer" charge to push the other TNT blocks forward. If you do this, be sure to use a wooden fence to stop the TNT that is propelled forward. The propelled TNT will blast through other "water stoppers" such as a fence or sign. Since the fence also stops the force of the blast, the TNT ammo must be dropped onto something at least as high as the fence.
These same principles should apply for "people launchers" since I have been able to get the cannons to shoot me pretty high in the air. (Confirmed - Able to get 100 blocks straight up with 48 TNT. Just make sure to cover the top, except for the block where you will stand for launch.)
I have finally been able to test a TNT charge using facing rows of dispensers 5 long by 6 high (60 in total) as propellant, the trough filled with still water to the top, which is covered, and a compression charge of 3 TNT. I had trouble getting the last TNT dispenser to work (the one with the ammo) until I used 7 repeaters set to fully delay - instead of my usual 6. In other cannons, I had to empty the columns nearest the dispenser to get it to work.