Original muzzle loading barrels made before 1830-1850 were all made of metals other than steel, and all were welded in some way.
The weld either went straight up the barrel as a straight seam, which was always mounted along the bottom of the barrel, or it was a spiral weld as GH stated.
However, a good inspection with a bore light will normally show any intrusion into the metal inside the bore along that weld line. A small intrusion which is detectable visually will often reveal a fault in the weld.
While the faulty welds have always been of major concern to gun scribblers, I have found the faulty breech plugs of more concern in real world restorations. Often a charge was left in the breech for decades and the corrosion hides many defects and causes others. That is especially true when the breech plug was not of high quality when installed.
It was not until the 1830s that fixtures were devised for deep drilling of solid barrel stock creating a seamless barrel.
I have seen several muzzle loading guns made with brass barrels, and they were quite popular. I once inspected a brass barreled rifle using a barrel that was a perfect square bar. Many naval arms were specified to have brass barrels and many entire issues of martial pistols were brass barreled. So if you run across a barrel that is not marked steel it does not mean the gun is useless.
I would hate to place anyone in danger, but I also wish to inform as well as possible. I have the freedom to experiment in many strange aspects of BP use and have learned that the limits of metal are greater when using BP than one might imagine.
I have made and fired BP guns constructed from many oddball sources, including water pipe, automobile axles, steel bar stock, brass stock and a couple of cannon made from pickup truck drive-shafts. Also made a nice 3" mortar from gas pipe, with an old wheel rim as the base plate.
I also once did an experiment to determine how much pressure a section of copper water pipe would withstand. That experiment was conducted by hammering one end of the 1/2" copper pipe flat and folding it over on itself several times, then drilling a touch hole near the folded end. The device was fixed to a 2x4 with a hose clamp and fired using canon fuse for remote firing from behind a secure barrier, to which everyone scurried with great haste!
The charge was black powder and a cloth patched .50 round ball. We raised the charge at each firing. I am not going to tell you what charge finally blew up the pipe (not knowing the age or strength of the pipe you have access too), but it was far more than I had expected and we got several shots off before the copper pipe gave up the ghost.
I once (20 years ago) had a modern BP side lock rifle brought into the shop with the complaint that the gun "puffed".
I informed the fellow that black powder was supposed to "puff".
He replied that it was "puffing" out the top of the barrel!
I inspected and found that someone had drilled and tapped the barrel for a scope mount and had drilled completely through the barrel. When this fellow removed the scope he had 4 holes 1/8" diameter puffing smoke out the top of the barrel. He had been shooting it that way for a couple of years.
I suggested that he dispose of the rifle, or replace the barrel, because I was not going to touch it for repair.
He scoffed that that idea and went across the street to the hardware store where he purchased 4 screws to cap the holes in the barrel. I never heard from him again, and did not read of anyone blowing themselves or their body parts up, so I suppose his method was a success.
(I also once had a modern shotgun brought in with holes for scope mounting drilled completely through the chamber area. It was still shooting but the shells would not eject due to the rough edges of the scope mount screws protruding)
I was working as "range safety officer" at a national BP shooting match and noticed one of the trap shooters was venting smoke out the side of the right barrel on his muzzle loading SXS. Yes, it was an original gun with twist barrels. It seems that he had been shooting this gun in that condition for nearly 10 years and no one had made any issue of it.
I called him off the line and blacklisted the use of that shotgun and nearly faced a lynch mob over the issue! Fortunately the President of that organization was working RO on the next section of the line and we called for arbitration. After the head safety officer pushed a matchstick through the decomposed side of the barrel they declared the piece unsafe and sent the shooter looking for a new set of tubes.
The thing was, (extremely scary thought) he had been shooting the gun in that condition for an indefinite period!
I have seen a lot in my days of shooting, reloading and repairing. I have seen enough to know I do not want to push my luck. And I have seen enough to know how far I can push my luck in a survival situation.
What it all comes down to is that you do what you have to do, and pray it works.
http://en.wikipedia.org/wiki/Siege_of_Boonesborough
Boone's brother Squire Boone was known as an inventor. He fashioned a makeshift wooden cannon, reinforced with iron bands, which was fired once or twice at groups of Indians before it cracked.