my concern is that the barrel idea is OK to heat a garage ie not continual useage ..
if you are planning on heating an entire house and burning as I do, about 5 full cords in a season, then the thickness of the metal of a barrel is insufficient to withstand the very high firebox temperatures ...it will get brittle and then fracture ...you'll be able to poke a hole through it with a ballpoint pen
The commercial boxes are burn rated for 20 years ...very heavy gage metal.
Mine has been used for 15 years and no sign of any deterioration.
You might get away if you could line the barrel bottom half with thin firebrick, or else lay a curved piece of sacrificial 1/16" sheet metal there.
Those fireboxes used to cost under $200 ...no idea the current cost ...but compared to a $2000 fuel oil equivalent ..a rather small investment to gain efficiency.
I had tempered glass firedoors installed at one time, but they would break from the heat ...some of the glass fell inside on the coals ...I have a sample of it as a curiosity....the glass melted into a 90 degree curve ...that is HOT!
My grates are constructed from 1 3/8" solid billets ..after three years, they are starting to sag ...I have seen them glowing red.
Do NOT underestimate the amount of heat generated within the box when you damper it down.
Stew