Springfield Armory during Mr. Lincoln's war went to great lengths to get good quality wrought iron for their barrels. During that period these were roll-welded from skelp, a process still used through mid-20th century to make "black iron pipe"
I do not know how they decided what iron was good, and what not. I do know that the guys with knowledge and experience in these matters have been dust for quite a while now.
Harpers Ferry Armory had fairly lousy iron and welded their barrels by hand in, say, 1830. Loss in proof at Harpers Ferry was as high as 25%, loss by triphammers at Springfield about 10% then. Do not know what it was during that unpleasant affair between the states.
Yup, wrought iron has silica slag in it. Wonderful stuff under the hammer.
Y' might want to read Harper’s Ferry and the New Technology, Merritt Roe Smith, ©1977, for some detail on barrel making.
Oh, BobintheWoods, are you really going to forge an actual gun barrel & put gunpowder in it?