Here is an exciting innovation combining the advantages of a rocket-fired combustion box with the practicality and efficiency of a wood-fired barrel oven.  This idea was developed by Flemming Abrahamsson of Fornyet Energi in Denmark.  See also his brilliant Rocket-Fired Griddle Oven Design, which further combines a cooktop griddle to the configuration.

This is an important evolution in the barrel oven concept because it both improves combustion and does away with the need for the additional hardware beyond the barrel of a firebox door, grate and ash drawer.  Notice the very simple yet clever sections sketched at 1 and 2 which demonstrate how the flue path is baffled to improve heat exchange.

Flemming has agreed to provide these drawings and the photos of a build sequence for free in order to maximize their potential reach.  You can contact Flemming for project specific design/consultation and also for barrel oven hardware in Europe.