Thanks to everyone for your encouragement and kind comments.
I took the plans from Antique Ornance Publishers (No 57) and scaled everything down to .40 scale. Bitch of a job.
Boomlover: length of tube is 27.4", bore is 1.76; golf ball = 1.68", that's .40 scale of the 4.2" 12 pounder cannon ball. Bore is slightly smaller than the 4.60 original 12 pdr, that should be 1.78. There's lots of windage in the original design! Breech is 3.60. tapers to 2.70 at muzzle. Wood is mahogony.
Cannonmn: I don't have a boat yet. I figure I'll ship out on a tanker, off Somolia--maybe we'll get some real action! 2 lbs of 00 buckshot would duplicate canister, to scale. Whatda think? Wanna go along? Really, it's intended to sit on top of a bookshelf. I'll shoot it, but it's really an "executive mood indicator".
Cat Wisperer: it has a loop under the barrel that started as a 2" piece of 1018 rod, turned down, milled with a flat on one side and fitted on a 1/4 piece of steel plate 2.00X2.60. 45 degree bevel cut on 2 sides of plate fitted into a dovetail under the barrel. Everything's welded.
Boomj: Golf ball + 1/4 cup of 2f may not have much recoil at all. 2 lbs of buckshot might have some recoil. And you're right--it has a loop.
Double D: Constructed of high pressure seamless tubing, 1.750 id, 3.75 od. Breachplug is 4" rod, turned down to fit 1.750X6tpi threaded, screwed into counterbore and breach face rounded from that. Cascabel screwed on, threaded with 7/8X6 acme tap.
Everything took a long, long time. Required me to learn how to operate a lathe and a mill from scratch. Those compressor nuts are hand made from 3/4" rod, and 1.75" rod, holes drilled on an old indexing machine! Tapped with a 7/8"X6 acme tap!
A machinist friend finshed the barrel for me. Moved from my former location, lost access to the machine shop I was using.
We're going to make another one, if I can find another piece of seamless tubing, 3.75od X 1.75id. Or, 1"id--we'll make a heavy (rifled) boatgun!
Regards
Shotgun