Coaxial

In a coaxial configuration, the barrel extends into the center of the chamber, and the breech end is near the chamber endcap. Coaxial cannons can be made shorter and more compact because part of the barrel is enclosed in the chamber. However, this also means that the volume of the enclosed barrel is subtracted from the total chamber volume, so a longer chamber is needed than equivalent over/under or linear designs. In combustion cannons, another advantage of the coaxial design is that the cannon can be easily breech-loaded when removing the endcap for venting. Coaxial pneumatic cannons almost exclusively use a custom built barrel-sealing piston or diaphragm valve design, since they are perfectly suited for this configuration. In long chambers, internal barrel supports may be needed to keep the breech end of the chamber centered. The part of the barrel inside a pneumatic coaxial launcher is subjected to outside pressure, it should be noted that the tolerance for this can be lower than the pressure rating of the pipe. There has been at least one incident where a thinwall barrel has collapsed from the outside pressure.