Copper pipe and fittings are popular spudgun construction materials due to their common availability in DIY stores. It is especially popular in countries such as the UK where pressure rated plastic piping is only available by order.
The primary advantages of copper pipe are that it is rated to much higher pressures than PVC, and has better failure characteristics due to it's high malleability which results in splitting rather than shattering. This makes it well suited for making high pressure pneumatic cannons and hybrids. The pressure ratings have a safety factor of around 7.
Copper piping is typically very smooth and consistent, which combined with it's high rigidity makes it exceptional barrel material. The consistent internal diameter also makes it popular for construction of combustion cannon fuel meters.
For comparable sizes of PVC it is around four times the price and considerably heavier, it is therefore only a viable alternative to plastic for small calibers of around 1" or less. However, DWV copper pipe and fittings are pressure rated! They are just rated to a lower pressure. DWV copper pipe and fittings are much cheaper than pressure rated ones. Pressure ratings can be found here
For comparable sizes of steel it is has a lower pressure rating and again is more expensive. However due to the thinner walls it is comparatively light and it is cheaper to make non-mechanical joins with.
Types & Use
Copper pipe is usually joined by sweating, using compression fittings, or Copper epoxy. Compression fittings are much more expensive than soldering ones but require minimal tools to use (a wrench or two), copper epoxy uses the same solder fitting as for sweating but replaces the solder to make the join therefore requiring no blowtorch.
The threaded fittings for copper pipe are made of brass or less commonly, corrosion resistant/coated steel as they have better mechanical strength than pure copper.
Copper pipe comes in two tempers, annealed and drawn. Annealed pipe is very soft, is sold in coils and can usually be bent by hand. Drawn pipe is rigid, sold as straight lengths and will flex rather than bend (to a certain limit). Bending drawn pipe without causing it to flatten out requires bending springs or mechanical bending devices which force the pipe over curved forms to create the desired shape without unwanted deformation.
Drawn pipe is the most useful in spudgun construction for chambers and barrels, since it will keep its shape and not get damaged as easily. It is also rated to about twice the pressure as an annealed pipe of the same size. Small bore annealed tubing has a place in areas such as pilot tubing from the main valve to the pilot valve where it's higher malleability allows for pilot valve positioning.
Due to it's relatively thin walled nature, potentially dangerous sharp ends can exist on pipe ends that have not been cleaned, and anybody using it should be aware of this.
Cutting of copper has a high attrition rate on rotary multi-tool abrasive discs, which will wear down at a high rate, and break apart regularly. Copper pipe should be cut with a hacksaw or tubing cutter.