Thanks for your reply.
Unfortunately I am not in the USA, I am in New Zealand ( a small country where they make movies like Avatar and Lord of the Rings) . We have some restrictive gun laws but the minimum bbl for shotguns in NZ is 12 inch not 18.
We are also allowed suppressors, as our focus is not self defence but hunting. There is no NFA to worry about, just no autos or pistols.
I was thinking of 0.729, 12 inch and a 1;20. I am glad 3 inch would be sufficent for subsonic, as I can use a standard breach.
I guess any faster twist than 1;20 would be counter productive.
The good news is that at 12 inch could get 2 barrels out of a blank.
Thanks again. will get cracking and send some photos soon.