Hello and welcome to NitroExpress!

You should edit your personal information to reflect the region or city you live in.
That's going to help the community here steer their responses and suggestions to you in the right direction.
What part of The States do you live in?

Are you considering this on budget concerns or as a hobby project?
Are you an experienced machinist and metalsmith?
Have you rebarreled and chambered many other rifles in the past?

If budget has anything to do with this and you want a hammer double rifle, look here.

There you'll see some used guns from a old, established, high quality rifle builder.
He's taken some very nice old european doubles and converted them to rifles -- including properly regulating them over a single set of sights -- by way of lining the original shotgun barrels with rifle tubes.
I handled a few of these this weekend, they're quite nice especially considering the price.
The images on his site are minimal, but I'm pretty sure he can take more photos of these and email them to you if you're interested.

The price to build your own conversion would likely be similar or more than the purchase price of these double rifles.

--Tinker