Disclaimer:
The materials I use for the construction of my launchers is tried and true to not fail when not abused. However, the manufacturers do not intend for their pipe products to be used in this manner. There have been reports about accidents with these devices because incorrect materials and/or construction techniques were used.

Accidents can happen if these are misused, and I take no responsibility in the misuse of my products. Never use one of these launchers in the cold. PVC becomes brittle at cold temperatures and can break easier. I generally follow the rule that if I need a coat, it's too cold for shooting.

Treat these as you would a firearm; never point it near something you would not want to completely destroy - especially living things. Never leave a launcher loaded and/or fueled when not in use. Never EVER for any reason look down the barrel of a spud gun.

The following is an excerpt from the Spud Wiki. Special thanks to SpudFiles.com.

Spudguns can be very dangerous if built or used improperly. However, if they are respected, they can be great fun.

Launcher safety

• Use good, safe construction methods and materials.
• Follow the rules of basic gun safety: never point the cannon at anything you don't want to destroy, and treat it as if it's loaded at all times. Never look down the barrel.
• Always make sure you are firing in a safe direction that is free of people, pets, livestock, cars, houses and other property. Use a solid backstop or make sure you have a clear landing area.
• Don't look into the chamber of a combustion cannon that has failed to fire. Fresh air can make a rich gas mixture lean enough to ignite.
• Don't use PVC cannons in cold weather. Sub-freezing temperatures makes PVC brittle and increases the chances of catastrophic failure.
• Don't use a cannon that is cracked or heavily scratched, or has been dropped on a hard surface. Joints may be weakened and the pressure ratings on damaged parts cannot be trusted.
• Don't use excessive pressure in a pneumatic cannon. PVC pipe is not rated for pressurized air because of the shrapnel risk, so use in a pneumatic gun is considered to be outside pipe specifications in the first place. Never exceed the pressure rating of the lowest rated part.
• Don't use overly powerful fuels in a combustion cannon. Solid fuels and very reactive gases such as hydrogen or acetylene are not suitable for use in spudguns. Don't use oxygen to enrich the mix in a PVC cannon.
• Use hearing protection when firing loud combustion and hybrid cannons, especially in an enclosed space. Use eye protection if there is a chance of splinters or other debris from the projectile hitting the target.

Don't shoot living things

A typical spudgun is capable of launching a potato weighing 50-150 grams a couple hundred yards with the muzzle energy of a powerful handgun. Longer distances are easily achieved with rocks, PVC darts, golf balls, and other aerodynamic projectiles.

This energy, even when carried by a projectile as fragile as the humble tuber, is easily capable of killing someone, or at least breaking bones or putting out eyes. As to property damage, most launchers can put a vegetable through a piece of 1/2" plywood, and many can put it through 3/4" - which is very significant considering the construction of most residential buildings.

The same applies to short/and or small-bore launchers too.

Despite their great potential to do harm, it is not looked upon well if you (attempt) to use your launcher as a hunting instrument. They are often hard to aim, leading to a low probability of a clean (humane) kill - plus; it is illegal to do so.

Some people don't take the extreme danger of a projectile fired from a potato cannon seriously enough. There are frequent news articles about people injured by potato guns firing projectiles other than vegetables, or people being accidentally shot while handling the gun.

Accidental discharges are the most likely cause of injury however some individuals lack the mental capacity to keep their guns pointed away from others at all times. When a potato gun is pointed at a human being the dangers are extreme. Do not under any circumstances let the barrel of a potato gun point towards another human, even if the gun is not loaded.

The bottom line is: don't aim a spudgun at anyone, at any time, even if you think it's not loaded.

Ammo Safety

Some advise against shooting bolts, coke cans full of concrete, and other hard objects out of your gun; saying things along the lines of: "Anything that is much harder than a spud can cause grievous bodily harm if shot at another person, or any animal for that matter." ...in actuality, even a tater will cause "grievous bodily harm" if shot at another living being. The danger lies in an increased chance of ricochet when using sturdy ammo - use common sense, and don't fire non-shattering ammo at close targets.

Some caution should be taken with shooting loose sub-munitions, specifically gravel or stones, with a sabot behind them - there is a significant probability of them jamming up, and the barrel being overstressed when the sabot tries to keep moving through the blockage, leading to a failure. Always wrap your lose projectiles in something - paper towels, old socks, and plastic bags are all acceptable.