A wall thickness of .020" at 15" would not concern me much after reading Sherman Bells articles in the Double Gun Journal regarding the loss of pressure at various points down bore. However, the thickness you mention at 10" and 12" would make me very careful in my choice of powder selection...charge weight...and payload.
I, and this is only me, would certainly want to keep the chamber pressure around 5000 psi. and use a slow burning powder to achieve that.
Regarding Black Powder, again referencing Sherman Bells articles, most of the loads he tested with Black Powder were not to different from the results he had with some of the smokeless powders.
All this being said, if I had another 12 gauge gun that would fit the uses that this new gun was to be used for, I would suggest the use of tubes.
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