Safety
As with anything, safe operation and construction of any fun device must be observed.
Combustion Safety
ALWAYS use appropriate pipe and fittings to construct your launcher. This includes all SCH-40 solid core PVC fittings and pipe (even DWV fittings), all SCH-40 ABS (cellular core inclusive), and all metal fittings and pipe. DO NOT use cellular core PVC for a combustion launcher, and DO NOT operate a PVC cannon in cold temperatures.
NEVER use oxygen-enriched fuels in a combustion launcher (such as the output of an oxy-fuel torch), and never use solid propellants.
NEVER look into the breech of a fueled launcher.
Pneumatic Safety
ALWAYS use pressure rated pipe and fittings. Fittings and pipe will have an ASTM rating which will tell you if they are pressure rated or not.
ALWAYS seal all leaks, especially in the pilot of a QEV or piston valve.
NEVER use pure oxygen or reactive gases in your pneumatic.
ALWAYS handle a pneumatic carefully when pressurized.
Hybrid Safety
ALWAYS use pipe and fittings rated for the maximum theoretical combustion pressure produced by the highest mixture you plan on using.
ALWAYS treat a fueled hybrid with the utmost respect, even if your ignition is completely disconnected. Even at low mixes, a burst disk failure can launch the projectile at high speeds.
NEVER operate a hybrid without calculating the peak theoretical ignition pressure of the mixture you are using.
Basic Safety
NEVER point a launcher at anything you do not intend to destroy.
ALWAYS wear ear and eye protection when operating a launcher.
NEVER operate a launcher intoxicated.
ALWAYS supervise other people when using your launcher.
ALWAYS use an appropriate range for your launcher. Be sure of your backstop and what the projectile will hit if you miss your target.