If you want something with the ability to first shoot a big hunk of lead and then a charge of shot wouldn't it be the easier to just get a SXS 10 gauge or 12 gauge w/ 3.5" chambers and load one barrel with a slug and the other with shot?

If you want to build this as your project fun gun then go for it but personally I doubt you will have many takers unless it is pretty inexpensive.