I'd be somewhat reluctant to use old wrought iron for a barrel. I'd tend towards either mild steel - the regular bars and rods available at steel yards - or what blacksmiths call pure iron.
Mild steel is iron plus 15/100ths of one percent carbon (or thereabouts). It was a replacement for wrought iron when steel making overtook wrought iron.
"Pure iron" is more like 5/100ths of one percent carbon, or less. It isn't as readily available as mild steel, but it forges more easily and forge welds more easily. Blacksmiths like it for decorative work. Look on the ABANA website for sources.
High carbon steel would be inappropriate - overkill for the strength requirements, hard to forge, and requiring careful heat treatment at the end.
The reason I am reluctant about wrought iron is twofold: it is a nonuniform manufactured product, and old iron, especially structural iron, can have flaws and inclusions in it.
Wrought iron is wrought (hammered) from lumps of iron mixed with slag. The slag mostly gets squeezed out, but some remains as longitudinal fibers. Wrought iron is ductile, but it is only really good in tension end to end (in the direction of the fibers). If you take a long flat bar and wrap it into a barrel (as they once did) it will be relatively weak in terms of resisting pressure from the inside. Think Elmer Fudd with his gun blowing into a splayed out bunch of strips.
If the wrought iron is good and the weld is good, then it can take the pressure of a black powder explosion. However, if there is a flaw in either, boom. Bad boom, not good boom.
The building supports were generally made of what was called "merchant bar," a less refined type of wrought iron. It was going to be used as a 1 1/2" square or larger, always in tension, so who cared about coarse grain or a few flaws? I have forged the stuff, and it was prone to splitting. The quality is extremely variable.
Old wagon tires might be better. Old hoops from wooden farm silos sometimes are wrought, but you'd have to either used them spirally as skelp or build them up. The problem is that planes of rust can work their way into the structure of the iron over 150 years. The spiral technique was common for damascus shotgun barrels, and it made sense. The fibers of the wrought iron ended up going around the barrel like hoops, in tension.
But still, rather than gambling on used wrought iron I'd use modern, uniform material.
I remember reading about a feu de joie fired at either Mt. Independence or Fort Ticonderoga in 1776. About 100 men fired their muskets/fowlers/whatever and three muskets blew up. So it went in the days of wrought iron.
If you do use wrought iron for a barrel, proof test it from a distance several times. And then a few more times.
