There are many types of valves available for spud guns.

Air Inlet:

Schroeder (tyre) valve-Great for small guns, lets you disconnect pump.

Quick Disconnect: Used on larger guns, for use with a large air compressor or a CO2 system.

Main:

Ball valve: Used as a main valve on low power setups, also used to isolate different parts of the spud gun. Advanced spudders use the ball valve as a piliot valve due to its high flow. Stay tuned for a 400 psi CO2 powered air gun! It will use a ball valve as the piliot and a QEV for the main valve. If you use a piliot operated valve like a modded sprinkler valve, QEV or piston valve try a ball valve as the piliot valve. For valves up to 1 inch use a 1/4 inch valve, for larger use 1/2 inch. For 4 inch or larger use 1 inch.

Piston valve: The gold standard: Very good performance, costs the most if bought, difficult to make.

Sprinkler Valve: A common choice, only plastic available, triggered by electricity or modded into something like a piston valve except with a diaphragm instead of a piston. Only available in plastic.

QEV valve: A store bought piston valve, must be found online, the very best.

Additional:

Safety valve: Lets air out if pressure gets too high. A must for PVC guns. Also required on all guns if using a pressure regulator as on a CO2 setup in case the regulator quits.

Pressure regulator: Reduces the pressure in a CO2 tank, paintball HPA tank or scuba tank to pressures a spud gun can use.

BONUS: Pipe materials.

Steel and Iron: Very high pressure rating, wide temperature range. Ideal for high pressure spud guns or CO2 setups because CO2 makes the gun very cold. Expensive, heavy and hard to work with but legal for compressed air applications.

PVC: A common choice because it is cheap, light and easy to work with. It shatters when it fails and the pressure rating is quite low. It is also brittle. It is illegal for compressed air applications, so DO NOT USE PVC PIPE.

ABS: The benefits of PVC, just it does not shatter, it splits. Much more expensive than PVC for pressure rated. ABS sewer pipe should not be used.

Copper: Benefits of steel, just it has to be soldered. It is lighter, but not rated for as high pressure and very hard to work with.

Other pipe materials are used but very uncommon.

DOUBLE BONUS:

Good and bad examples.

GOOD EXAMPLES:

http://www.kiestu.com/videopage/on/eTL1OqQdrHY.htm..http://www.spudfiles.com/pneumatic-cannon-database...

http://www.airgununiverse.net/qevairgun.html-QEV

BAD EXAMPLES:

http://www.spudfiles.com/pneumatic-cannon-database...-Ball valve

https://www.instructables.com/id/Air-Cannon-v3/-Soda Bottle Chamber

DO NOT USE PVC PIPE OR ABS SEWER PIPE FOR COMPRESSED AIR!!!