Tested and Confirmed.
I had to downgrade one of my worlds because the game crashed if I got too close to the centre of the map due to the fact that there were 2 item frames hanging on the wall.
Oh... Okay.
Thanks for letting me know.
Tested and confirmed.
I have tested this cannon in multiple versions and the problem only exists in Minecraft 14w17a.
I have also tested this cannon ussing different materials and they all have the same result.
BTW why is it so important that you can seal the cannon without the TNT phasing though?
I suppose that is true but this also occurs if you land on the original block but even if you are close to the edge.