I made a dry ice cannon when I worked for a dry ice distributor in the south end of Columbus. For the most part it looked just like a spud gun but it had no igniter. I used a section of 4" necked down to 3" barrel. For the explosive charge I used a 20.oz bottle with dry ice nuggets/water and the projectile was a tennis ball. When it went off it sounded like a cannon. The projectile left the barrel so fast, you could only see the ball as it dropped in the distance.
I took it to my brother in laws farm to shoot it, I would say it shot a 1/4 mile easy.
The only problem with this design is if the bottle does not explode you need to leave the gun alone for a day or two so you don't get hurt unloading it.