I found the following description of barrel-making in an 1818 work by Sir Richard Phillips (1767-1840),
The book of English trades, and library of the useful arts (London, Stereotyped by G. Sidney, for R. Phillips; published by J. Souter, 1818). It may be that this is already a well-known passage. It may also be suspected that Phillips is not entirely to be relied upon in matters of detail. In any event, I though it of enough interest to share here.
A facsimile of Phillips' work, containing the complete article on the trade of "Gun Maker", from which this is an excerpt, is available from the
Internet Archive:
"To form a gun barrel in the manner generally practised for those denominated common, the workmen begin by heating and hammering out a bar of iron into the form of a flat ruler, thinner at the end intended for the muzzle, and thicker at that for the breech; the length, breadth, and thickness of the whole plate being, of course, regulated by the intended length, diameter, and weight of the barrel. This oblong plate of metal is then, by repeated beating and hammering, turned round a cylindrical rod of tempered iron, called a mandril, whose diameter is considerably less than the intended bore of the barrel. The edges of the plate are made to overlap each other about half an inch and are welded together by heating the tube in lengths of two or three inches at a time and hammering it with very brisk but moderate strokes upon an anvil which has a number of semicircular furrows upon it, adapted to the various stages of barrels. The heat required for welding is the bright white heat which precedes fusion, and at which the particles of the iron unite so intimately with one another that when properly managed no trace is left of their former separation. These heatings and hammerings are repeated until the whole barrel has undergone the same operation, and all its parts are rendered as perfectly continuous as if it had been bored out of a solid piece. For better work, the barrel is forged in separate pieces of eight or nine inches in length and then welded together, lengthways, as well as in the overlapping. The other mode being the easiest and quickest done, is the most usual.
The barrel is now either finished in the common manner or made to undergo the operation of twisting, which is a process commonly employed on those barrels which are intended to be of a superior quality and price. This operation consists in heating the barrel in portions of a few inches at a time to a high degree of red heat; when one end of it is screwed into a vice, and into the other is introduced a square piece of iron with a handle like an auger, and by means of these, the fibres of the heated portion are twisted in a spiral direction, which is thought to resist the efforts of the powder much better than a longitudinal one."
