Hello Gramps-

Welcome to the list.
Get some photos of that double rifle up here on the site so we can see what you have there.

I have two pinfire bore rifles, they're sixteen bore double rifles and I shoot them both.
You're going to need to cerrosafe cast both of your chambers including an inch or so of the rifled bore, and also slug the bores at the muzzles.
You really need to know just exactly what those chambers and bores measure out to before you can proceed. It's going to be very helpful to know what the rate of twist is too in order to factor everything when it comes time to choose projectiles. It'll also be helpful to know how long the barrels are and how much the gun weighs. That will help determine what kind of load the gun was originally intended to shoot. The chambers may have been lengthened at some time since the gun was originally finished to clean up pitting or damage to the origin of rifling, or in some previous attempt to 'get the gun to shoot'

I'm a machinist and have lathe turned cases for both of my guns, I run roundball in both of them. These guns were originally intended to use paper cases. I've made my cases with fairly thick walled necks to account for that. Some pinfire rifles were set up for thin brass cases, most of the ones I've seen were set up for paper though.

Guage those barrels, let us know how your chambers and bores mike out, we can take it from there.

And get us some photos of that old honey!

--Tinker