I have built two of them,m and wrote an article about the 1st some years back for M/B.
here's the website where I have some basic drawings on how mine worked.
http://marina.fortunecity.com/seaview/283/mypage.html
They are fun to think about, fun to build and at first fun to shoot. But no matter how you make one, it's going to be heavy. I pruned the barrels back to 28" and the thing still wears you out dragging around all day hunting. I guess what I'm saying is, they ain't all they are cracked up to be.Mine were smoothbore/rifle, both of them.
John Elliot shot a swivel shotgun down at Friendship for years and years, and it would probably be about as light as you could make one. Even he said it was a chore to carry hunting.
To keep as light as possible, you want the biggest bore you can get in the smallest diameter barrel you can get. I used a .45 in 13/16 and a 62 in 1". A pair of Ed Rayl's 3/4" barrels in 36 or 40 (if he would make one) would be good.The second one I made for a friend, used tapered barrels in .54 cal and that helped some.
I've always been enamored with multi barrel guns and made a double flint shotgun years ago with the help of Bill Brockway's book. I still have in my mind an over and under flint double rifle or shotgun with the locks parallel to each other. Ive seen them made with perc locks and the one lock having a long nosed abortion of a hammer, but it looked like hell IMO. I have the idea for how the reciever would work in my mind but need to buddy up to a machinist to get it made. To dream to dream !