As hard as we try, we just can't get that "5" to turn into a "3", so we will have to be contrarians here and go with the 5" Armstrong 40-pdr 1.32-Ton BLR which, in it's bore dia., is listed as a 4.75 ", 22 Calibre long tube, Naval rifle. Although some dimensions are exactly the same as the ones on our 20 pdr gun drawing, others seem be be much larger/longer. Woops! Forgot about Allen's first drawing hint! The wall thickness at the muzzle would be a bit thin if this were a 40-pdr, about .150", a wee bit thin is certainly an understatement. I guess we have to defer to Boom J. on this one, but what about that 5" bore dia.? Maybe I need new glasses.
Thanks for the explanation, Allen; it sounds like you have plenty to do and hopefully you can give us a blow-by-blow of this winter-season build with in-process pics. It's hard to beat Boom J's wonderful pics. That is, of course, the carriage Lamb's Confederate artillerymen loved to hate as they chased Federal gunboats away from the beach adjacent to Fort Fisher, North Carolina. They had Whitworth tubes though, 12 pdrs, 2.75" calibre. In the Whitworth System, the 5" rifle was an 80 pdr; such are the vagaries of British ordnance.
Thanks for the update, Allen.
Mike and Tracy