Smokeless powder is not an explosive but a flammable solid. In open air it burns fiercely but will not detonate. It is much more dense than black powder, and much more powerful. If you took the volume suitable for black powder in a cartridge and filled it smokeless, you would have created a bomb. Shooters and reloaders who were used to black powder had a difficult time creating enough fillers to take up the space difference. Powder manufacturers even developed special semi-smokeless and smokeless powders that were high-bulk and could fill the powder space in shotshells.
Even this was not enough for some shotguns. For centuries barrels had been made by the Damascus method. Two pieces of metal, one each of iron and steel, would be heated and hammered flat. The iron could be a common grade of scrap iron such as reclaimed horseshoe nails. (Could I make this up?) In an age of horse-drawn everything but railway carriages, leftover bits of horseshoe nails were common. The steel could be a good Swedish or German steel, or any tough and hard steel. The two flattened and clean pieces would be laid together, and the real hammering would begin. Heated and folded, heated and folded, the steel and iron would be layered many times, like a pie crust. Eventually the layered metals ended up as a rectangular bar. The bar was then heated and twisted in a spiral (with a lot more hammering) around a steel rod called a mandrel. As it was twisted the edges would be “welded” together. I use quotes because that was the term used at the time. The edges were actually forged together. The mandrel was then slid out of the finished tube. Once cool, the tube was filed, polished, reamed and turned into a barrel. For its time, and when used with black powder, a Damascus barrel was quite strong. For those who like the looks, it was also beautiful. The swirling, repeating pattern of the iron and steel would take blueing differently, and the pattern was obvious to any who saw it.
The pattern is also the weak point of Damascus. The edge between the iron and steel is a joint. Rust has a way of attacking joints. The joint is also weaker than the iron or steel even if it isn't rusted. The burning rate of smokeless powder created two problems for Damascus barrels. The peak pressure of smokeless is greater than that of black powder, and the rate of onset (the steepness of the upwards curve to the peak) is much faster. Smokeless hits harder and comes on faster than black powder, enough so that damascus barrels can rupture. Even if the pressure peak is kept down to that of black powder, the rate of onset is too quick and can still rupture a damascus barrel. The lesson learned then, and one that holds true today, is do not fire smokeless shells in a Damascus barrel.
To speed up the process, manufacturers looking towards mass production instead took a long flat section of mild steel (it was all mild steel in the 19th century) heated and fed it into a forge. The forge would stamp the steel into a “U” which was then finish-forged over a mandrel and the single seam “welded” shut. Much faster and cheaper to make than Damascus, it also produced a barrel more amenable to lathe-turning to final shape. Not until deep-drilling of steel rods became common did shotgun barrels lose their seams. In deep-drilling, a steel rod is drilled its length, and then reamed and polished to produce a barrel blank. Compared to the earlier forged barrels, a drilled barrel is absolutely uniform as a produced item. That uniformity greatly aided the development of reliable and safe breech-loading mechanisms. The new steel barrels were often referred to as “fluid steel” barrels. Not because the steel was more flexible, but because it came out of the crucible liquid, and was poured into molds to make ingots that were forged into bars, rods, rails and I-beams.
Damascus had been the mark of a fine gun for so long that some makers would take their new and stronger fluid steel barrels, and etch a Damascus pattern onto the outside. And you thought appearance for appearances sake was a late-20th century innovation.
While the strength drawbacks of Damascus eventually led to its demise as a barrel steel, the American emphasis on mass-production would have put the skids under it even without the advent of smokeless powder. (Damascus does seem to be making a comeback as a knife blade material.) As if the weakness wasn't enough, producing a Damascus barrel was more expensive. The many thousands of hammer blows it took could only come from a skilled worker, and his output was limited to a few barrels a day. Once created, the Damascus barrel had to be hand-filed, bored, reamed and polished. The iron and steel were each of a different hardness. Trying to turn a Damascus barrel on a lathe as a production process would have been a nightmare. With orders for thousands of shotguns, Winchester, Remington, Savage and the other large manufacturers could not even consider making barrels from Damascus.
Modern designs sometimes mean non-traditional approaches. Unlike the elaborate ejectors found on some doubles, this pump has an ejector that is just a sheet-metal tab in the path of the empty hull.
Damascus held on for a while in doubles, where mass-production was slower in arriving. One of the tricks of producing a double was in “regulating” the barrels. If you fasten the barrels together and to the shotgun parallel to each other, the center of the patterns of each barrel may not agree with each other. Trying to hit a fast-moving duck or goose is hard enough without having to remember “the left barrel is high and the right barrel is left” or some other mantra. Regulating the barrels in a double involves test-firing the shotgun before it is finished (termed “in the white” because it hasn't been blued yet) and adjusting the fit of the barrels to each other. The muzzle ends of the barrels are bent, tweaked and wedged until the center of each barrel hits to the same point at 40 yards. With all the hand work involved, the extra work of Damascus barrels is a small additional cost.
In the United States, test-firing shotguns (and rifles and handguns, too) is performed by the factory that made them. Considering the loss of business in the old days, and the potential litigation today, each factory is greatly interested in making sure each shotgun is properly designed and manufactured before it leaves. Things were (and are, for as long as they continue to have firearms) different in Britain. Centuries ago the established gunmakers found themselves competing against imports and cheap, shoddily-made local junk. To protect their customers (and to keep out the junk) they established “Proof Houses.” The London Proof House received its charter in March of 1637! The Birmingham Proof House received its charter in 1813. The job of the inspectors at a Proof House was to pass judgment on the safety of each gun presented. Once it had been measured and gauged, it would have a “proof load” fired in it. The proof load was a standard-size shell loaded to greater pressure than regular ammunition. After digesting the proof load, the shotgun was again measured and gauged. If no critical dimension had changed, it was deemed to have “passed proof” and was stamped as such. Even imported firearms had to go through the Proof House in order to be sold.
The British have never been into the high-pressure magnum ammunition that Americans seem to be enamored with. With sufficiently thick chamber walls and a low-pressure operating limit, British shotguns would last forever if properly treated. There was so much safety margin that many British Damascus-barreled shotguns were re-proofed with smokeless powder. Again, do not fire smokeless shells in a Damascus-barreled shotgun, even if you find British nitro proof marks on it. The operating pressures it was proofed to are probably lower than the standard pressures current American shotshells are loaded to.
The purpose of a proof house is to prevent poor-quality goods from being sold. And to protect a country's internal production capacity from imports. The “W” or “double V” is proof this piece passed inspection and test.
In the space of a generation, shotgun hunters went through a gantlet of choices. From muzzle-loading, percussion shotguns with outside