rocket mass heater: heat riser material- brick or metalbestos chimney pipe?

I'm considering using 8" metalbestos double wall chimney pipe for the heat riser in an 8" system and wonder if anyone has experience with using this material and/or comments on the pros and cons of using it as opposed to a brick heat riser.  I'm also wondering about using a smaller, 25 gallon barrel instead of a 55gal drum.  It seems like this is common in many of the videos/photos posted and I'd like to know what are the main considerations here.  I'm building the system inside a greenhouse with the main goal being to heat a cob bench for bottom heat for the flats, more so than radiant heat from the barrel itself.  Because I'm not so interested in the heat coming off the barrel, what size/configuration of barrel/heat riser would work best?  Thanks for any input and my apologies for any repeat questions.

Metalbestos?  What's that?

Sorry for the delay in response - I was sure that folks more knowledgeable than I would respond.

Oops, sorry.  Metalbestos is a brand name, I guess, for an insulated, double wall, stainless steel chimney pipe.  I have seen reference to a similar product being used as a heat riser, but haven't heard much of an evaluation of its performance.  In the meantime, I've decided to go with a brick heat riser, so never mind...  Thanks for getting back to me though.    I'll begin putting the stove together today!  I can't wait to see what mistakes I'll make!

Well, for the sake of anybody else that might end up with a similar question:  I think those triple wall things are probably fine for a normal chimney, but I think they would really suck for use inside of a rocket mass heater combustion chamber.

they work great for heat risers and we use them often. saves time and then being SS they last for a good long time.

drill a couple holes through there outer wall near the bottom so if they have any water in them they can shoot the steam out.

pros they heat up fast and are time savers with good durability.

cons they are expensive and don't hold heat like a masonry heat riser so you have to stick a bit of paper further down the burn tunnel to warm then up.

sorry to be a little late on the response all; its the beginning of the workshop season so we are a little busy.