The CrMo machines better than mild steel DOM seamless. Either is OK for smoothbore barrels. Make darn sure that DOM mild steel tubing is actually seamless. Some is welded and then drawn. Also make sure there are no flaws in mild steel tube. Proof testing would probably reveal flaws, but by that stage, a lot of work has gone into the barrel. I've made smoothbore barrels from mild steel tubing, both tapered round and tapered octagon to round. It can be done, but a better grade of steel is much more machinable. The difference between 1015 and 1137 is night and day. Barrels for a double would have to be tapered, if the gun were to handle like anything other than a club.
A RIC in .38 Special. Interesting. The chambers could be reamed, sleeved and chambered, or the sleeves could be machined to fit the .450 chambers, then reamed for .38 Sleeving the barrel would be straightforeward. Going to a smaller caliber would make the sleeving easier, less worries about thinning the barrel and cylinder walls. A .38+P might be a really bad idea in a converted RIC. I would be inclined to use a slightly shorter cased wildcatted .38SP. Of course, if the gun never fell into the hands of anyone who didn't undertand that modern spec. ammunition would be a bad idea in an antique, it wouldn't matter.