At least it was soft dog poop and not something hard that would do a lot of damage.
Potato cannons, if built right, can be very powerful.
In high school I build an air-compressed potato cannon (or, as I affectionately called it, 'The Produce Accelerator') with a pneumatically-actuated sprinkler valve. At 120 psi I shot an enormous nail with duct tape fins through two phone books, an SAT study book, and a box of sand about 20 inches long. It pierced all of those and kept flying into the woods behind my house to never be seen again.
It could shoot a golf ball upwards of half a mile with only a 5 foot barrel. One time I attached a 10 foot 1/2" barrel to it and I made a AA battery break the sound barrier.
With a 5' barrel that was 3" in diameter I would shoot soda cans (full!) nearly vertically. They would become a tiny black speck in the sky and land about 20-30 seconds later with a giant explosion (shook them up beforehand