Some double shotguns are beautiful works of precision and art while otheres are at the very least on the not hideous side of the spectrum, well I've decided to bastardize this long and glorious tradition by attempting to make a rough and dirty workhorse of a gun. I've made some basic .22 zip guns that didn't blow up in my hand and a 12 gauge piece of sh*t lead pipe gun that really should have looking back, so I think I'll jump into the deep end with this project. I'm thinking of using a 12 gauge coach gun as a template for this, exposed hammers and single action inlet into an oak stock. The barrels will be made of some 4130 tubing, just one question, will .82 inside diameter be acceptable for this or did I measure wrong? And after some research I found out that the barrels for most shotguns are just soldered together and the ribs are just used as reinforcing, which leads me to another question. Do I need to do anything to the barrels to re-temper them after I'm done soldering? The older guns apparently use soft soldering instead of silver solder which might mean I can avoid screwing up the temper too bad, but I'm still not entirely sure if I should just use silver solder instead for strength. Any suggestions would be nice and lord knows I'll need it.

It's safer to buy used shotgun barrels and just silver solder  (high temp) them together with a rib

For the receiver, you really should stock with 4130 or 4140 for the steel. this is the common steel used for most gun builds (unless Stainless is used), and it will allow for easy heat treating.