I had trouble getting my hands on a tank with a 30" diameter vs. wall thickness I was looking for, so I had one of the big tank building shops in town roll a piece of 1/4" steel for me. It was not very cheap, but it got me going.
Depending on availability and what size you are going for, large diameter pipe is also a consideration. I think after about 24" dia. it goes up to 3/8 wall by default, which might be better or worse for a cook chamber (better heat retention, but longer to get up to stable temp and harder to cool off if you get too warm), if you can safely handle the extra mass of the pipe and are equipped to weld something that heavy. (my 0.02 anyway)
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My motto on building smokers: “It does not matter how slowly you go so long as you do not stop” ~ Confucius
If i had a piece rolled to around 30", Would 3/16" steel be fine to use? Or should i go with 1/4". This will live on my patio, but may occasionally go to mom and pops for family get together's. So weight is a small factor..
I used to date a girl that was flat on both ends.... Don't get me wrong, she got the job done... But the curves are better flow! In all seriousness I don't see any difference between my pit (curved tank) and my buddy's flat. Go with 1/4" the cost difference isn't that big.