My advice would be to save some time and spend a bit more money on firebrick. Also ditch whatever design you have and go with a true and tested design found for free from Peter Van den Berg's site
http://batchrocket.eu/en/. This design is the best and fail safe. As John said, the firebox should be insulated. Do not use metal pipe as the heat riser within the drum, not only does it need to be insulated--this is one of the most important parts of the rmh workings, but the metal will burn out. You need firebrick or perlite riser to offset the temperatures between the inside and outside of the riser. The horizontal run is very small and the vertical pipe does not count towards your total. Also, the space between your riser and the barrel is small. A recommended 12" is the minimum for a batch style rocket stove.
I'm also going to mention that the stove appears to be very close to the wooden stairs. Cob around the barrel will get very hot and could build up heat igniting a wood surface. If you do place a stove in this spot, make sure to put a layer of fireblanket between the combustibles and the stove/pipe to prevent catching fire.
I see a lot of issues with this layout and the material--I highly recommend going to peter's website and reading through the entire thing to get a better understanding of a batch rocket mass heater. I would also recommend purchasing a copy of the Wisner's book found here
https://www.amazon.com/Rocket-Heater-Builders-Guide-Step/dp/0865718237/ref=sr_1_1?ie=UTF8&qid=1540218790&sr=8-1&keywords=rocket+mass+heater and/or a copy of Ianto Evans book found here
https://www.amazon.com/Rocket-Mass-Heaters-Ianto-Evans/dp/B01FIX87WG/ref=sr_1_2?ie=UTF8&qid=1540218842&sr=8-2&keywords=rocket+mass+heater and giving them a really good read. Knowledge is power and lighting your house on fire would suck.
Cheers.