In a coaxial configuration, the barrel is placed in the center of the chamber, and the breech end is near the chamber endcap. The design makes a very compact and rather nice looking launcher, since part of the barrel is enclosed in the chamber. Also, this is useful for diaphragm valves or piston valves. 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 in equivalent over/under or linear designs. Mounting the barrel like this is usually done by means of a modified bushing.
In combustion cannons, another advantage of the coaxial design is that the cannon can be easily breech-loaded when removing the endcap for venting. However, it can be a pain to vent and mix the combustion products/fuel since there usually isn't much space for chamber fans or traditional spark gaps. A spark strip is well suited for use in a coaxial chamber.
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.