Hi Tristan; Big Welcome to Permies! And A Big Welcome to the wonderful world of rocket science!
I think that your piece of pipe is rather thick.
As you thought, it would take time to heat up.
The other possible problem, if you are planning on a piped exhaust thru a mass, is stalling the heat flow.
On a much better note. The motor oil barrels you have are exactly what we use here. There is no, too thin. The heat dissipates fast. The most you would see is a glowing orange circle directly above the riser.
It would be best if you could locate one with a removable clamped lid. However any 55 gallon (200 liter) size barrel will work.
I would like to tell you about another option. All brick bells as a stratification chamber rather than a piped mass. And a batch box style rocket rather than a J tube rocket.
Here is Peter Bergs website explaining all about batch box's.
http://batchrocket.eu/en/
They do require some metal fabrication, but its not to difficult.
Another option would be to build a J tube and plumb it into a stratification bell.
As you can tell there are numerous options.
Here is another website from master builder Matt Walker.
Matt has designed super efficient riserless stoves and he sells the complete plans with full personal support from him.
Here is his website
http://walkerstoves.com/index.html
As a side note. Each Wednesday at 11 am pacific time, Matt hosts a live chat with folks from around the world. You can ask him questions and receive his
answer live!
Here is the chat site information.
https://www.youtube.com/user/broaudio. All his previous chats are also available at any time.
We have a happy crew of rocket scientist eagerly awaiting your questions!
We will leave the light on, come back anytime and ask questions!