I have always wondered how you get the two barrels of a double rifle to shoot to the same point of impact. Usually, a quick Internet search gives the answer to any question, but not this one.
I found pieces of the answer in various places, so decided to bring them together in one place to see if I could make sense of it, and share some of my journey through this fascinating topic.
Part 1 – The Beginning
At first, I imagined the two barrels were parallel. As a result, they would shoot parallel, with each barrel sending its bullet along the same trajectory, separated by the thickness of the barrel walls. In effect, there are two rifles glued side by side. In that way, the shots should always fall together, above or below the point of aim depending on the distance to the target. If the shots are not falling together then the barrels must not be parallel.
But then I read that the barrels are angled slightly. So much for the first theory.
Then I imagined the barrels working like the wing guns on a Spitfire. Each barrel would be angled inwards slightly towards the sight axis, so the bullets would converge at one distance, which could be altered by adjusting the angle towards or away from the sight axis – a sharper angle in towards the sight axis would zero the barrels closer and a shallower angle would zero the barrels further away. In either case, the shots would diverge from the point of aim if your target was further away than the distance at which the barrels were zeroed, or the target was nearer. This is known as “point harmonisation” and was adopted by the RAF in mid-1940. Before that, wing guns were angled to converge in a box ie guns had both vertical and horizontal dispersion[1].
In W W Greener’s book The Gun and its Development[2] he states:
“The workman called in the Birmingham district the “barrel-filer”, and in London the “barrel-maker”, takes the tubes, and for a double gun joins two together, fits top and bottom ribs, the lumps, loop, etc., required for the breech action. The most important point is the jointing of the barrels, by filing flats on the inner sides in order to get the tubes closer together, and at such an angle to each other that the axes, if continued beyond the muzzle, will converge at sixty feet beyond. If the barrels were not closed in they would shoot “wide” – that is right to the right, the left to the left, of the mark at which the gun is aimed. This is due to the fact that, being juxtaposition, the inner side of the barrel, reinforced by its neighbour, does not expand equally with the outer side; barrels placed one above the other, instead of side by side, shoot high and low instead of right and left. The breach end of the barrel being of necessity stouter than the fore-part, the gun would be unwieldy unless joined in. One barrel being brazed to the other at the breech, the thinnest sides are practically reinforced by the metal of the neighbouring barrel, so that the inner side is in reality stronger and less likely to burst than the outer and thicker side of the barrel.”
However, he appears to be speaking of shotguns in this analysis, rather than rifles. He only turns to rifled arms later in the book. He has this to say on “under-and-over rifles”[3]:
“It was once thought that the difficulty experienced in making the ordinary double-barrelled rifle shoot both its shots to centre could be overcome if the barrels were placed under and over, instead of side by side; for in the ordinary double-barrel rifle the shots are generally thrown outward – the right to the right and the left to the left – and this was put down to recoil pulling the gun over at the moment of firing. To prove this the author made several under-and-over rifles, but found the two barrels still shot away from each other, the upper one high and the lower one low, proving clearly that there was some cause other than recoil – probably the unequal expansion of each barrel due to the proximity on one side of the barrel attached to it.”
The first quotation seems to support the “wing-gun” approach, with the barrels angled inwards to converge at a certain distance. I thought the second quotation was odd in dismissing the role of recoil in explaining the differences in point of impact of the bullets from each barrel. The expansion of each barrel (presumably as a result of heat) must surely be a smaller factor than the recoil produced by even a small double rifle cartridge.
Perhaps Greener was too quick to dismiss recoil as a factor – I will look at this in more detail in Part 2.
[1] See https://en.wikipedia.org/wiki/Gun_harmonisation
[2] Cassell & Company Ltd. 9th edition at p. 263
[3] Ibid at p. 656
