What do you do if modern factory ammunition results in shots crossing-over or in widely spaced shots? Or ammunition is no longer commercially available for your rifle?
You can deal with the first situation by having the rifle re-regulated with your preferred ammunition. The solder holding the barrels together is softened and the barrels are adjusted as described above until the barrel shoots with your preferred ammunition. The barrels are then re-finished. The cost is not insignificant.
You can deal with both situations by handloading ammunition that provides the correct barrel time for your rifle. In other words, the same barrel time as the ammunition used to regulate the rifle in the first place.
If the bullet speed is too low, and so the barrel time is too long, the bullets will leave each muzzle after it passes through the point of aim and the two shots will be wide apart (but not crossed). In other words, the muzzle of the right barrel has moved above and to the right of the point of aim by the time the bullet leaves the barrel. The muzzle of the left barrel will have moved above and to the left of the point of aim.
If the bullet speed is too fast (and so the barrel time is too short) the bullets will exit each muzzle before it passes through the point of aim. The two shots will cross, with the point of impact of the right barrel being to the left of the point of aim and vice versa. The muzzle of the right barrel will be pointing below and to the left of the point of aim when the bullet leaves the barrel. The muzzle of the left barrel will be pointing below and to the right of the point of aim when the bullet eaves the barrel.
Your starting load will probably produce a widely spaced (but not crossed) group, because the speed is too low. As you increase the load, the speed will increase and the shots should converge. Consistency is important, and a standing shooting bench will provide both comfort and consistency. It also matches the position in which the rifle will be used in practice.
Of course, this all assumes the barrels were regulated by the maker in the first place. No amount of handloading will produce a group from a rifle that has not been regulated.
Graeme Wright’s book, Shooting the British Double Rifle, is now in its third edition and is an indispensable guide for anyone shooting double rifles. He provides this very helpful diagram (at p. 166) to illustrate this process:
Cross-firing usually indicates the velocity is too high. However, W. Ellis Brown notes in Building Double Rifles on Shotgun Actions, 2nd Edition at p. 151 that it may also be caused by a load that does not produce sufficient recoil to move the barrels the necessary distance. This reduced load would also produce a lower velocity. As a result, the starting point should be the published velocity for the original ammunition for the rifle.
With older rifle information there is also the problem that accurate measurements of velocity were difficult, and much harder for customers to verify. There must have been the temptation for the maker of a proprietary cartridge to “over-state” the velocity in order to claim an advantage over his competitors!
But what about over-and-under rifles? In Part 5 I will look at how regulation applies to those rifles …
