PVC pipe and fittings are the most common material for spudgun construction. PVC is cheap, lightweight and available in a wide range of dimensions and ratings. PVC is usually joined by solvent welding, which is easy to do and requires no special tools. Threaded fittings are also used, but then solvent welded to the pipe. PVC is easy to machine, and parts are sometimes customized to fit a certain purpose.
PVC is usually colored white or shades of gray, but transparent PVC pipe is also available. It's more expensive than normal PVC, but is somewhat popular in combustion launchers, where clear parts will let you see that happens inside during the combustion process.
One of the main disadvantages of PVC is that it becomes brittle in cold weather, which may lead to chamber failure, especially in pneumatic launchers. When PVC fails under pressure it breaks into sharp shards, which are propelled at high velocity. Because of this danger PVC pipe is not rated for use with pressurized gases, and the use in spudguns is to be regarded as outside of specifications. Shrapnel from failure of PVC can be fatal. The reason PVC does not shatter with water is that water is not compressible, so water stores very little energy. Air is compressible and stores lots of energy. As a result, pressure rated ABS pipe is a better choice for a spudgun, as it is rated for compressed air. You can find it online or at an industrial supply store. There have been reports of serious injury and death from exploding PVC pipe, even at low pressures. Alternative materials, such as ABS, copper, and steel are much better choices.
Schedule is a method of denoting pipe dimensions with a standardized set of nominal sizes. Note that this is a size convention, and does not necessarily denote a pressure rating. Harvel has tables of dimensions for SCH40 and SCH80 pipe sizes.
SDR (Standard Dimension Ratio) pipe has the same O.D. as Schedule pipe, but with different pipe I.D. All SDR pipe in a series has the same ratio of O.D to wall thickness throughout the sizes, and thus has the same pressure rating. All SDR pipe is rated for pressure, but because it often has relatively thin walls (in smaller diameters), it should be sleeved or otherwise reinforced before use (in some cases). Harvel has charts of dimensions and pressure ratings for SDR pipe. (Larger diameters of some SDR pipe actually have thicker walls than Schedule pipe. For example, SDR21 pipe 5" and up in diameter is thicker than Schedule 40 and is used in well casings. SDR21 2.5" (Tennis Ball Barrels) are thick enough to not require sleeving in barrel use). SDR21 1.5" Golf Ball barrels are not rugged and sleeving is recommended.
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