Here's what W. Greener had to say about joining barrels in his 1835 book (Peter Hawker wrote the introduction):
ON PUTTING DOUBLE BARRELS TOGETHER.
The art of putting double barrels together seems to be but little understood, even by the first makers. It is a rare occurrence, indeed, to find double barrels properly put together, having neither too much nor too little inclination at the muzzle. I shall, therefore, take the liberty of making a few observations on this very important point; also, on the proper elevation of the rib. The trade have to thank Joe Manton for this, as one of the best inventions he ever brought forward. Were proper attention paid to the construction of these ribs, they would be invaluable, yet we every day see them bungled by blockheads in a way to make every judge of good work wish the bungler in Siberia.
Having then got your barrels ready to put together, and cut to the length you intend, it remains to be determined what inclination they should have. Be they long or be they short, they should, to be perfect, come to a point at a given distance, if for general sporting. If, however, you are putting together a large pair for wild fowl shooting, the inclination must be altered to suit the greatest distance to which it is intended the shot should be driven. There is a great diversity of opinion as to the proper inclination of a pair of double barrels. It is needless to state the precise distance at which the converging lines drawn from the centre of each barrel, and indicating the inclination of the barrels to each other, should come to a point. If we take the point of convergence of those lines at 2 1/2 yards, it will follow that at 40 yards each barrel, were it fixed in a vice, would throw the centre of its charge six inches on the opposite side of the mark, or body fired at; but if the gun be fired from the shoulder, the recoil will invariably cause the gun to swerve outwards, so that at that distance, she will never fail to throw her shot in a good direction for the mark or bull's-eye.
The subject may be understood by the following observations. All tapering substances, when laid together, were the taper extended, would come to a point at a certain distance. Gunbarrels are made to taper towards each other, and some more than others. To make them uniform, it requires that they should be reduced or flattened to allow the thick or heavy end to joint closer, to allow that point of convergence to be extended to a greater distance. If then, we take two barrels, two feet eight inches long, and having a solid substance of metal at the breech of 3/16 of an inch each, and is 1/16 at the muzzle; it requires the difference 4/16 to be multiplied 45 times (there being that number of lengths in 40 yards) to ascertain what distance the points of the different lines are from each other, which will be 11 4/16 inches, or 5 10/16 from the centre or line of sight. If you wish to reduce it from the centre, you have to join the barrels so much nearer at the breech ; or should the inclination be too little, the muzzle must be jointed closer. As, however, all guns are now made very heavy at the breech, they very seldom require any closing at the muzzle, though it is customary to do it, and to a great extent; but it is owing to the ignorance of the nature of shooting.
Having satisfied yourself that your barrels are set correctly, the next thing to ascertain, is the proper degree of elevation. Different lengths require a difference in the height of the rib. A greater height is also required for a person accustomed to use a crooked stock; and less height for one accustomed to the use of a straighter one ; and so on. Few barrels are to be met with in which the elevation is sufficient; a species of innovation much practised by gun makers of the present day, whatever merit there may have been in the original invention, there is none in " the improvement," as they term it. Take any of the modern barrels, and calculate what is the real elevation of them, and you will find it is not equal to the distance that charges will droop at 40 yards, when we consider the very large charges of shot that many are accustomed to use, without a corresponding quantity of powder. It remains then to be decided, what elevation a gun should have for that distance.
I have tried the experiment some hundreds of times with guns of all descriptions, both with a rest and from the shoulder, though standing as firm as possible ; and by turning quickly round, and firing (as I might do were a bird to spring in a situation where I could get only a snap shot) against targets, such as are used in military ball practice, being about six feet high, and by means of which you could perceive where the body of the shot had struck. I have also fired against the steep sides of those beautiful sand banks of which we boast on our own immediate coast, and on which, from their smoothness, you could tell every shot that had struck them. Those sand banks I found much superior to sheets of paper for trying guns, the marks being all visible at once, so that it is easy to define the outside limits of the charge. As to the last method I have tried, no one whatever has, for the time I have had an opportunity of trying it, fired more shots, or, as it has been told me, wasted more powder and shot by firing on the water, both at living and dead marks. To sum up all, my conviction is, that almost all guns charged, as it is the custom, with heavy charges of shot, droop full twelve inches at forty yards, though by using small charges of shot you will find them to be thrown much more correctly than the heavy charges ; so that it is possible to make a gun too high on the rib for a shooter that thinks more powder and less lead preferable to much lead and little powder, for believe me nothing is more injurious to the shooting properties of any gun than too great a weight of projectiles. Powder may be compared to the strength of a man. Give him a stone four pounds weight, and desire him to throw it—mark its flight; give him one half that weight—attend to its flight; one will cut a segment of a circle— the other a parobolic curve, keeping a direct line until its initial velocity is gone, and then cutting a part of a circle, though it has gone twice the distance, and with twice the force; thus proving, that if weight is to be thrown with only a certain propulsive force, there must be elevation to carry it to the destination intended, though at a much less speed and force than if that weight were one half less. Thus a gun maker can only construct the elevated rib by guess, if the gun be not ordered, or the maker unacquainted with the sporting ideas of his customer. It is nonsense to lay down a scale of charge, and say, let the shooter stick to it. It were as possible to turn the course of the Thames as to convince some old shooters of the erroneousness of their ideas respecting the propriety of having large charges of shot. The obstinacy with which they continue to use very large sized shot is a proof of this. They will tell you that you talk like a child should you affirm that small shot, with a moderate sized gun, will kill equally as well as large. I can assure thein that I can kill, and
have killed scores of times, the larger sea-gulls with No. 6, when they have gone away with two or three charges of Nos. 1 and 2 fired at them ; and when they have been skinned I have found the large shot in a body of feathers just under the skin as large as a marble, while the small shot had not only cut clean through the feathers, but considerably through the body. I am quite convinced that moor puffings, or any other close feathered sea-bird, can more easily be killed by small shot, say No. 6 or 7, than by larger, if the gun be of sporting dimensions. But to return to the elevation.
It being difficult to determine, on any scale of elevation, while such a diversity of opinion on charges exists, I must endeavour to convince gentlemen of the superiority of my plan, which they will find fully treated on under the head Shooting. However, I think the elevation I have given, will be found to be as near what is requisite as possible, if we continue to load as heretofore; if reduced charges of shot be adopted, a less elevation will suffice. To ascertain what elevation at the breech, for the above scale is requisite, take the thickness of the breech and muzzle, and multiply the difference by as many times as there are lengths of your barrels in the 40 yards, and you will then ascertain what elevation they give of themselves ; and to make up the difference wanted, must be the elevation of the rib, which may be calculated in the same way as the barrels; the length of the barrels being the only way of obtaining a correct idea of the height required. If you should be making Woodcock guns, less elevation is required, the distance of shooting being shorter. In large guns, a greater elevation is required. I believe, however, Colonel Hawker has fallen into an error, when he says that long guns require a greater elevation than short ones. But does not a long gun keep the shot more together ? Is not more force generated ; and is not the initial velocity greater than in a short gun ? If these be facts, why is more elevation required, if the shot do not droop? I apprehend, the Colonel means, if the same height be required to be given above the mark; nothing can be plainer than this—that if one pair of barrels be four inches longer than another, and the elevation the same, there cannot be as many lengths in the 40 yards of the longer barrels as of the shorter; and hence the difference, when multiplied. I think, therefore, he cannot have taken into consideration, the superiority in their shooting; for there cannot be a doubt, that, if a gun keep the shot together longer, it cannot require that allowance for drooping, which a shorter gun does.
There is another objection which I have to make to the present mode of putting barrels together, and then I shall dismiss this part of my subject. I object to making elevated ribs hollow.
Whether they are made hollow for the purpose of saving iron, or of making the barrels lighter, I never could learn, but I hold the practice to be highly injurious to the shooting of the barrels. If lightness be the only advantage, it is a trifling one, as it does not lessen the weight more than three ounces ; and should any water in washing get into the inside through any imperfection in the soldering, it will not fail in a short time, to create such a rust, as will, sooner or later, inevitably destroy the goodness and safety of your barrels. It is on the score of lightness alone, that the practice can be defended. The shooting powers it cannot increase, being more liable to increase the expansion than if they were solid.
