Surely Doc by now you are ready to move on to something less amateurish that that thing. Besides it replicates no pre 1899 design, that I am aware off.
I have some complete plans for long Cecil I would gladly share with you. Yes a post 1898 design, but I would provide a copy of the ATF letter making it antique.
http://www.gboreloaded.com/forums/index.php/topic,243416.0.htmlYou have demonstrated the skills to build such a gun, why not go for it?
I am building mine from a 20 mm barrel, you could build it from a .50 cal barrel or even a smooth bore. You could have a rifled breech loader that shoots surplus .50 cal slugs. You could also get a rifled muzzle loader or shot gun barrel from Gun-parts company.
Long Cecil-Williams-727 [Public domain or Public domain], by Book: Williams, Gardner Fred, 1842-1922, Photos not attributed, from Wikimedia Commons.
By the way it looks like now my build will not get started until 2014.
There some other good breech-loader designs you could do that look like cannons and not some kids toy. The French 75 that Dom build as a muzzle loader, could easily be made as a breech loader as it was original. We have also had a number of Whitworth's built here. Plans for the Whitworth are avaialble from AOP.
Get a copy of the
Abridged Treatise on the construction and manufacture of ordnance in the British service (Google eBook). This book is full of cannon drawings that you can scale from as well as drawings showing how these guns were built.
Here is an example
Any of these guns will look great and shot good. Build one of these and it won't look like a gun that some kid built, like that ebay gun. You will have a gun that you can be proud of and pass on to future generations.