David-

As soon as I saw you were on the way to opening a hunt101 account, I went there to see what that site was like, and to see if I could find your new account.
There was a column of images noted as 'recent photos' or something like that.
I tried to open them (make them bigger) by clicking them, and failed -- the site wanted me to join.

I then right clicked on them, studied the properties of the images, and from the text part of the web address marked "location" I just cut and pasted into a fresh browser, omitting the "thumbs" and associated slash, then hit the enter key.

That got me these images at the size you see in my post.

So these barrels are fully rifled?
I strongly suggest blackpowder and roundball.
Make chamber casts of your chambers with Cerrosafe casting alloy, you can get it from brownells.com
Take soft, pure lead roundball of a size slightly larger in diameter than your rifles bores and with a hardwood mallet tap them into the muzzles, then with a long wooden dowel or cleaning rod tap them back out of the muzzles and measure the bore and groove dimensions.

Post the measurements here on this thread.
It might take a little while for you to get the cerrosafe alloy from Brownells.com, but make it happen as soon as you can (to satisfy my selfish curiosities) so that we can give you accurate counseling on cartridge case selection and preparation, projectile selection, and load information.

It's going to take pretty accurate numbers in order to get you started in the right direction.

What you have here is a very nice rifle, it should get back into the hunting field, and I for sure (and can speak for many of us here) want to help you get it running and back to the sporting life.

Also, as a side note, one of my 16b double rifles is a Mahillon.

Here's a photo of it.

--Tinker