No matter if you build and use a combustion gun, or a pneumatic gun, they still contain the ability to propel projectiles at extremely high speeds. One person has build a marble gun, that runs of pneumatics and can propel marbles at speeds in excess of XXX Hundred fps. That's right around 2/3 the speed of a .22 caliber bullet. However a marble contains anywhere from 3 to 5 times the mass of a .22 bullet, AND it's made of glass instead of lead. There are others Dan Thames that have built pneumatic cannon that can shoot soup cans of concrete nearly all the way through a house. No matter the potato gun that you build, you MUST keep in mind that what you have, despite it's outward appearances is actually a Gun, a weapon, and you must treat it with the respect that they command.
There are a few basic, and obvious rules of handling and use that apply to potato guns. You should treat them just as you would a firearm. In case you have no clue as how to properly handle a firearm, I'll list a few of the basic lines of etiquette & handling: Do NOT point them at people, animals, cars. or buildings.
Now I understand that with most potato guns you have to actively do something to prepare them for firing, whether it be the injection of fuel (propane/hair spray), or the pressurization of the air chamber. And that thus requiring such, they for the most part will not be able to readily fire if a small child, or other unknowing person were to come along and pull the trigger. Another thing is putting them away loaded. Although it may seem kinda pointless, I have loaded the barrel on my combustion gun, and then put it away. I didn't notice it was loaded till the next time I went to shoot it. Rotten potatoes stink, smell, and are REALLY nasty & slimy to clean up. So if even just for not wanting to clean up the mess, don't put them away loaded. The bottom line comes down to this: Treat the potato cannon as you would a loaded and ready to fire gun. Don't point it at anything that you do not want to shoot & destroy/harm. Although they may seem like toys, they still have the ability to kill just like a firearm. Just use Common sense and try not to become a statistic, or get arrested while having fun.