Thought I'd share this rule with others who might want to suggest it to their own gaming groups. We decided in our group that Cannons are far too powerful in 8th Edition - the fact that they have a 89% chance of hitting (presuming the player knows what he is doing) is just insane (I've been told it's 89%, but I havn't done the maths myself). We came up with a simple house rule that still makes them very useful, but also stops them being sniper guns. The rule is based on the assumption that if they can pack too much gunpowder in - resulting in an overshoot - then you can pack too little and undershoot. the rule is simply you place where you would like the shot to go as normal, then roll a d6 with the artillary dice, on a 1-3 you undershoot by the artillary dice and on a 4-6 you overshoot. Obviously the "bounce" roll can still only go forward. This still leaves some skill to placing the ball, but lowers the chance to hit to something more in the region of 63% (according to our resident statatition), which even the players who can use cannons agree make for a more fun and fairer game.
Or simply go back to the pre-8th edition rule of guess+artillery die, then roll artillery die again for bounce distance. The misfire on the guess resulting in a roll on the misfire chart, a misfire on the second roll just meant no bounce, or what my group referred to as "stuck in the mud"
That sounds like a pretty nice solution actually. It would be hard to go back to the guess range option since everything is allowed to be premeasured these days.
I haven't personally had that much trouble with it, myself. How much terrain are you using? My last game against Ogres the guy had a lot of trouble getting good shots off because his massive troops were in the way; if the cannon had been in front of them it would have been mowed down by skinks. Remember that anything the cannonball hits and doesn't kill will stop it cold. Some obstacles can be helpful as well...
Place a marker to show where you want the shot to land, and roll an artillery die to see many inches too far it moves (if Misfire, roll on Misfire table). The cannonball bounces artillery die distance from the new position, and can only cause damage from this point. Does that give an 89% chance of hitting?
If you place it right (I think the optimum is 6" away from the front unit/model but may be wrong) and based on a unit being 3 ranks deap - or a monster base. Atleast thats what our resident statistician says - it goes up to something nearer 91% if you have an engineer apparently.