For a century now, barrels have been made from solid bars, bored and reamed, like this Browning barrel.
Many barrels, high-quality ones, were made by the Damascus method. Hand-forging a section of steel around a mandrel in a spiral pattern produced a good (for the time) barrel, but not one that could be mass-produced or produced to exacting, repeatable tolerances. The later method, employed by larger makers, of forging a barrel from a flat section of steel, punching it into a “U” shaped channel and then forging the seam produced more uniform barrels, but they were not much stronger. Without uniform barrels, you could not depend on the cartridges to seal the chamber on firing. Some early breechloaders, such as the Ferguson rifle, had a good seal. Developed just before the Revolutionary War, it used a spiral screw at the rear of the barrel. Turning the trigger guard rotated the screw down to expose the breech. It was accurate, reliable, sealed the breech well and too advanced for the British military. A later American attempt was the Hall. The Hall used a hinged and removable lock and breech assembly. By dropping each pre-loaded block into the rifle, a trooper could fire until his supply of pre-loaded breeches was used up. In a pinch, the removable block could even be used as a pistol of sorts. The problems with the Hall were fragility, poor gas seal, and the heavy weight of a supply of blocks.
Even from the earliest times, gunsmiths and inventors worked on breech-loading firearms. This is a breech-loading flintlock that uses iron or steel cartridges each with an integral frizzen.
The pinfire cartridge was an early attempt to perfect the enclosed cartridge. Each round has its own firing pin, which sticks up through a slot cut in the edge of the breech. The hammer strikes the pin, firing the cartridge.
Precision manufacturing allowed the use of cartridges made of brass instead of steel. Precision manufacturing also made the brass cartridges a tight seal against the combustion of the powder, increasing safety to the shooter. The impetus for the precision manufacturing was the American Civil War. The need for weapons, and ammunition to feed them, was greater than hand-production could supply. Machine-made firearms had the advantage of dimensional stability. That is, in order for all ammunition to work in all rifles of a given caliber, both the bullets and the bores had to be made to precise, and limited, dimensions. When a gunsmith was making his muzzle-loading shotguns one at a time a “12-Gauge” could mean his barrels were 12-gauge plus or minus .050” and no one would know. When reliable cartridges were designed and manufactured, a shotgun could not have a chamber smaller than the largest cartridge. If the chamber were too much larger than the cartridge, there would not be a proper gas seal. The advantage that troops using cartridge-firing firearms had over their muzzle-loading opponents was significant. A rifled musket could be loaded and fired four times a minute by a skilled soldier. He also had to stand up to reload. A soldier using a cartridge-firing rifle could fire at least twice as fast, and could do so, including reloading while prone. The Federal Army had to have cartridge firearms, and was willing to spend money to make the arsenals that could produce them. If cartridge repeaters had so much going for them, why was so much of the Civil War fought with muzzle-loading rifled muskets? Production, or rather, the lack of it. It does little good to equip an army with breech-loading rifles if you cannot provide them with ammunition. With arsenals set up to produce muskets and ammunition, the Federal Army would have been negligent not to use them. Because proven breech-loading weapons were available with ammunition to feed them, they were used. Some units even bought new designs out of their own pockets in order to gain an advantage.
Balancing the need for production against tactical advantage is not new. In 1543 English armories developed a method of making cannon barrels from cast iron instead of bronze. Cast iron is heavier, weaker and more brittle than bronze. And it rusts. When the tubes burst they shattered, creating casualties of the gun crew and adjacent soldiers. However, a cast-iron barrel can be made for a fraction of the cost of a bronze tube. Faced with the option of going to war with one company of artillery or four, for the same cost, what would you do?
After the Civil War the advantages of self-contained cartridges were so great that Colt did a brisk business converting cap-and-ball revolvers to fire cartridges. The same advantages applied to shotguns, and gunsmiths were quick to design breech-loading shotguns.
One design requirement of black powder flintlocks and percussion firearms was the need to keep the shooters face away from the breech. Unlike today, a shooter in the era before cartridge shotguns kept his head up, and away from the breech. To get the barrels up to his line of sight, the stock had to have an appreciable angle down, called drop. The drop in the stock created a lever to direct the force of recoil into the shooters face. Ever since cartridges have become common, stocks have gotten straighter.
Getting to today's plastic shotshells took quite a bit of work. Early shotgunners had a choice that many shooters today would think odd: The pin fire. In the early days of shotshell design, primers were not an easy thing to manage. Making the mixture (even coming up with an appropriate compound) was not easy. Getting enough into a shell to create complete combustion of black powder took more space than modern primers have. To maximize the use of the priming mixture, each shell had its own firing pin, resting directly on or in the priming pellet. The Lefaucheux system had many merits, primarily that it worked. Since shotgun makers were applying for design patents for breech-loading shotguns in the 1850s, the pinfire system came into common use and hung on for a long time. Remember, the shift to any new technology takes time, and in the pre-computer age it sometimes took a generation or two. As precise as manufacturing had become by the 1880s, primers were still expensive. If the priming compound was not evenly distributed in the cup, misfires and hangfires could result. I think pinfires also had a following (and for quite some time in Europe) for gunsmithing ease and shooter comfort.
And even after the pinfire was gone, the hammers would stay. Imagine yourself a gunsmith used to making a shotgun with external hammers (as black powder percussion and pinfire shotguns still would have been for decades by 1880). Or a shooter used to the visual safety margin cocked or half-cocked hammers represented. A shotgun that broke open in the middle and used self-contained cartridges was a shock for many older shooters in the 1880s. The hammers kept things familiar enough for both father and son to keep shooting even when the pinfire cartridge disappeared. Webly & Scott, shotgun and rifle makers in Birmingham, England, still listed double shotguns with external hammers in their 1914 catalog. I'm sure sales of hammer guns did not come back after the Great War. Anyone interested in shooting would not want to put up with such old-fashioned nonsense.
Military depots and gunmakers would test each batch of powder they received. If the powder did not register the proper power when fired in a test-gun like this, it would be rejected.
The shotgun shell was developed to hold a large amount of black powder. When smokeless arrived, the wad had to be changed to take the extra space left by the new more-compact powder. When steel shot arrived, there was plenty of room to adjust again.
Americans did not take to the pinfire in anything like the numbers that the Europeans did. I think it was the emphasis on breechloaders and ammunition capacity that crimped the pinfire's style. As much of a genius as John Browning was, I think even he would have thrown up his hands if Winchester had insisted on pinfire shells for a repeating shotgun. And if the ammunition companies were going to make centerfire shotgun shells for repeaters, then the makers of American doubles would just have to adapt to them. On both sides of the Atlantic, the pinfire was on the skids even before the next revolution, smokeless powder, came along.
Successfully developed by the French for their military rifles, smokeless powder was a wonder. It was compact, powerful, and produced hardly any smoke. It revolutionized military rifles and cartridges.