Compared to some other processes in the early days of gun making, rifling was not that laborious. Forging a bar of iron into a trough shape, welding it into a tube and then reaming the inside, straightening it, and then grinding/filing the outside profile of the barrel was far more work. Rifling was, however, something that had to be done slowly and carefully. The older way to do this was to have a spiral guide made of wood. A rod with a scraper at the end was dragged through the barrel, scraping one groove, and turned by the guide. The guide was indexed, turned to go from one groove to another. After every rotation the scraper was adjusted to cut a little deeper. It could only cut a little at a time- it was important for the groove to be even , and match the other grooves, and the best way to achieve that was to scrape off metal slowly. In the old days, before 1800, the cutter was shimmed up with thin paper, daubed with oil, and resharpened when it seemed to be getting dull. The process could be thought of as filing the rifling by hand, and so it would take a couple of days for two men to do a barrel, depending on how much daylight they had for working. You can still see them do this at Colonial Williamsburg.
This process was somewhat mechanized in the industrial age. A water wheel or a steam engine ran a line shaft in the shop, leather belts connected the line shaft to various machines, among them a rifling machine. It was still working much like the earlier one, except that the machine was now cast iron and steel, the wooden spiral guide was replaced by steel or iron sine bar, or rack-and-pinion with cast iron gears. The rifling rod was still scraping a groove in the barrel, but because it was pulled by machinery, indexed by machinery, and the rifling cutter was adjusted by screwed wedge automatically after every rotation, one employee at, say, Harper's Ferry Armory could tend several machines- so the time needed was greatly reduced. You can still see one of these machines operating in the demonstration shop at Harper's Ferry National Park (at least, it was still working not too long ago). But the whole process of making a barrel was also streamlined. Forge-welding the tube was done more quickly by running the tube through shaped rollers at welding heat, instead of incremental forge welds on an anvil., reaming was done by water or steam power, grinding and lathe turning as well. And there was machine production of other parts, as well, like locks. Even the gunstocks began to be shaped on a duplicating lathe.
In the later 19th c. there would be further improvements in that basic rifling process: the rifling cutter would be made out of high speed steel, that would not need to be re-sharpened as often as carbon steel, and barrels began to be made of steel which, being more homogeneous, machined somewhat more smoothly than wrought iron. They could be drilled, instead of welded. But rifle design changed as well. Simple muzzle loaders were replaced by more complex breechloaders and those replaced by even more complex bolt-action magazine rifles and then finally semi-automatic rifles. The advent of smokeless powder put barrels under much higher pressures. Bullets had to become jacketed, typically with copper, and the deep rifling of the earlier rifles was replaced with shallower rifling, with wider grooves. That presented an opportunity for a different approach to making grooves. Production at the Springfield Armory had not been abler to supply enough 1903 rifles in WWI, and at the outset of WW II there was a clear need to boost production of the even more complex M1 Garand. Along with a lot of other machining processes, hours spent in cut-rifling a barrel now began to look wasteful. The Springfield Armory tried using longer cutters, broaches, that could be pushed through the barrel by a hydraulic ram and essentially cut the two grooves of the rifling at one pass. Pratt and Whitney then created rifling machines that would use a hydraulic ram to force a shaped carbide "button" ( actually a short rod) through the barrel. The button had ridges which would push grooves into the steel of the bore. Naturally, the process took some development- copper plating the bore was needed to lubricate the button and keep it from jamming. But it was better, more trouble-free , than broaching. And compared to cut rifling, button rifling was quite quick- a button could be pushed through the barrel under a minute, as opposed to hours for a cut-rifled barrel. That became the standard process.
There were other ways developed to rifle barrels- forging the barrel down around a shaped carbide rod, using Electrical Direct Machining for removing metal from the bore. Button rifling is still the most common method. But cut rifling did not disappear. Some target shooters stoutly maintained that , among other things, cut rifling put fewer stresses on the barrel and so a cut-rifled barrel would shoot better. And so the method survived. There are still plenty of barrel-making companies that still do it: though, it's more likely that the process is now run with computer operated machinery, the gunsmith writing G-code to make it happen.
Harold Gill : The Gunsmith in Colonial Virginia
Merrit Roe Smith : Harper's Ferry Armory and the New Technology
James Howe: The Modern Gunsmith
William S Brophy : The Springfield Rifles