I built a prototype small stove with well insulated burn chamber, that burnt very efficiently, and used it for heating water quite effectively. I have never seen one with a riser.
In practice, what sort of temperatire can you expect on the surface of the Oildrum (riser)?
When I built a prototype, it burned at a very high temperature, but it went through wood pretty quick I thnk a bigger one would eat wood pretty quick. Is the idea you don't keep them burning all the time or is there a way to slow burn??
With a rocket mass heater, the idea is that you run pipes through a 'mass' which extracts all the heat and acts as a thermal battery, which gives off heat into the room for many hours. You never slow burn a rocket mass heater, you burn as fast as thoroughly as possible, but only for a short time. I think most people only burn them for an hour a day, but the mass keeps giving heat off for 24 hours.