A linear launcher (also called an "inline") is a type of cannon in which the barrel is directly in front of, and pointing in the same direction as, the chamber. Such a configuration is slightly cheaper than the over/under configuration, but only by the cost of two 90° elbows. The linear design is most often used in combustion launchers, and is considered the most basic launcher design. It's also the simplest to assemble, requiring no special tools or fittings.
The linear design is often considered slightly more powerful than over-under configuration since the traveling gases don't have to make a 180° turn, which slows the flow of gases. In addition, inline designs tend to be somewhat sturdier since the recoil forces are transfered linearly to the chamber of the gun. In an over-under design significant stress can be placed on the elbows as the barrel recoil energy is transfered around the bend to the chamber.