I've continued search for the answer to the question posed.
Thank you, Ron.

21 December 1861

"Anderson (Tredegars in Richmond, Va.) has recd orders to manufacture a large number of IX inch guns and 32 pdrs of 57 cwt. to be cast and made in all respects like the guns made by the U.S. Navy under contract of July 3rd, 1857. All founders object to turning the gun off, they want to cast to the next size."

John Mercer Brooke

J. Thomas Scharf mentions that Capts. Maury and Whittle request 11 inch bore for the gunboats proposed for building on the Pamunkey River.

Recommendation to Commodore Barron

May 11, 1861 (before the CSN existed)

I'd always been under the impression that tooling was a limiting factor in bore diameter used. This is, to some extent, true. Tredegars could bore to 24 inch diameter, but only to a depth for ordnance the length required for a mortar.

Dahlgren had been in ordnance for some period of time when the seccessions occured. The large (11 and 15 inch) bores were already inventory in 1861.