1. Get a friend to get in the Mortar.
2. Get in a vehicle and bumb into the mortar to knock it over while your friend is still in it.
3. Your friend needs to go into the Artillery Computer and start to click and try to fire around the area.
4. You might need the host to get in the Mortar but you should see your friend bouncing everywhere in the Mortar, or if he sets the range to maximum he might start sliding across the ground at ridiculous speeds.