It will be interesting to see the details of Abner's retort. It sounds like the more conventional type in which the feed-stock is in a completely enclosed container (except for vent holes, normally at the bottom, for escaping gasses) and the fire is built around the inner container.
I was surprised by Gary's design, and also by the "continuous feed" concrete hopper design, since these are open at the top and are fired directly. I imagine that making good charcoal in these requires good timing (capping off the charcoal at the right time).
Anyway, I am going to try a design similar to Gary's. However, I am going to have to modify it somewhat so that I can just use the materials on hand.
As I commented before, it appears to me that the outer barrel is mainly providing structural support in that design, and I intend to eliminate it.
Here is what I have in mind, unless some of you more knowledgeable people tell me it won't work.
I intend to use a single 55 gallon drum to hold the feed-stock.
I will put small slits in the bottom of the barrel.
I will cut the bottom off another 55 gal drum, along with 4" to 6" of sidewall.
I will weld this "second bottom" to the bottom of the first barrel, airtight.
I will put a 2" hole through the sidewall of the new section of barrel, and insert a 2" pipe through it, and into the space between the "new bottom" and the "slitted bottom." This will provide the restricted air supply. I will make the penetration through the sidewall airtight, and put a valve on the pipe so that I can close off the air.
I will fill the 55 gallon barrel nearly full with wood, then build a fire on top of the wood.
I will place a lid with a 12"-14" hole in the middle on the barrel.
I will place a chimney over the hole (probably 48" high, and rolled from a piece of sheet metal).
After the charcoal is ready, I will remove the chimney and hole-lid, replace it with an airtight lid, and close the air intake valve, making the entire assembly airtight. I will let it cool for 24 hours, then see what I've got.
Unless I have missed something, this will preserve the essential features of Gary's design while dispensing with the outer barrel.