Well on the Calliope the rockets were 4.5 inches, so 3/4 inch in 1/6 scale.
I have found a place that sells thin wall tube, total cost $82.00 for the tube.
The cheaper route is way to big 1" OD, when putting 16 across top would be 16" plus inches across turret on 1" OD, so I guess thin wall...