Those original rifle barrels were hammer forge-welded from a flat skelp of wrought iron. They started by putting that skelp on convex forms, heating, hammering, and slowly bending that skelp into a "u" shape. Then they put a mandrel inside that cupped skelp so they couldn't accidentally hammer it too small or flatten it too much in one spot. Then they continued to heat, and hammer it around that mandrel paying particular attention to where the edges of that skelp came together heating and hammering them sufficiently that it closed together with a hammer-forged weld. This was a seamless weld that resulted in a barre without any weak spots when done correctly, but you couldn't make any mistakes with the heat used. That hammering to shape and weld the barrel gave it the octagonal shape of the barrel. They would clean it up by filing afterwards but the intial shape was made by the hammering to turn that flat skelp into a barrel.
At this point they would knock out the mandrel and basically have a rifle-barrel blank. Next they would ream it out themselves or send it to a cannon boring facility if it was close. Reaming it out or "boring the barrel" cleaned up the interior of the barrel and accounted for the final caliber. You could get a number of different calibers from the same mandrel, depending upon how much reaming was required to make the interior smooth and straight. You might end up with .50, .52, or .54 caliber rifles from the same mandrel. That's why every rifle they built came with bag mold to cast the correct size lead ball for that rifle.
Once the barrel has been hammer forge-welded and reamed out, now it's time to do the rifling. Rifling was done on a long, wooden machine cutting one groove at a time, a fraction of a 64th of an inch (they couldn't measure 1,000ths of inch then) by adding shims under the cutter after taking numerous strokes in each groove and repeating the process with each groove the same number of times to produce a consistent depth. Often, the only metal part was the actual cutter(s) itself
Here's a video with the owner of Rice Barrels demonstrating it. It's a short video - less than two minutes, but gives you a good idea of what it took. His rifling machine has 3 cutters in it all in a row:
Every once in a while you'll see one of these in use at a Black Powder event. I saw one at work in Monroe, WA (greater Seattle area) at a weekend event. They worked on it all day both days and still didn't get the barrel finished before the event was over. Everything about making a rifle barrel is a very labor -intensive process. Heat management in the actual forge-welding step is particularly tricky because if the heat is not correct, the skelp won't weld correctly.
Since everything was basically done by hand, they made swamped barrels for better balance (a longrifle with a swamped barrel is a joy to use!). no such thing as nose-heavy with a swamped barrel. It wasn't until 1838 when Remington invented a long, hollow, drill bit that straight barrels became the norm because they could be drilled out of solid blanks. No one had been able to drill out solid steel blanks before that because the bit would get too hot and melt or lose its shape. By making the drill bit hollow and pumping oil through the center of it as it cut, they could actually drill all the way through a blank, eliminating the laborious hammer forget-welding process with a much faster drilling process. This also gave them the ability to consistently make barrels the same caliber.
This process of making a rifle barrel in the 1700's was why rifles were typically 3 to 4 times as expensive as fowlers or muskets.
