This response does not answer your question directly.
However, the link can take you to a thread that you, as a gent with "gun projects', will find very interesting.
http://www.nitroexpress.com - will take you to the parent BBS. Some sub-boards are visible to non-members. Membership will allow you access to more sub-boards. The (membership only) sub-board you want is "Building Double Rifles & Gunsmithing". The article/thread you want is ".303 British Double Rifle Project" - an excellent article complete with detailed pictures and well explained technique.
Depending on your practical experience, motivation, and equipment, you may just decide to do the double rifle project yourself. I lack the talent and motivation, but I found the author's comments on welding and bluing really interesting and valuable, IMHO.
Hope this helps.