So I’ve tried pretty much everything to make the kill balls and man cannons to cooperate (they just get sucked in and stay there instead of being launched). But then I randomly came across this wierd solution. If you take the unsc man cannon (invisible doesn’t seam to work) and turn it upside down, the kill ball will launch if you place it underneath (which would be where the blue launch animation is) the man cannon. The kill ball will then launch in the man cannon’s path. It’s hard to explain, just remember to turn the man cannon upside down (preferably a little bit over the ground) and then try it yourself.
Hope you make some awesome maps with killballs now 
Thanks for this! I’ve been playing around with kill balls and we’re able to launch them like you suggest but since the latest update it seems to have made the ball/cannon interaction worse. Have you experienced this too?