Classic416:
It is a commonly-held misconception that the flight-paths of bullets from each barrel of a double must 'cross' at some distance. In effect, they are regulated to shoot parallel. the reason for the confusion is simply that to shoot parallel, their bores must converge slightly in the rifle itself. This is because each barrel recoils slightly away from the other on firing, before the projectile leaves the muzzle.

A properly regulated rifle will form a useful composite group with the bullet strikes from the left barrel on the left side of the group, and those from the right barrel on the right side of the group, all the way to maximum possible range.

At the time these NE double rifles were built, standard 'factory' ammunition was available. They were simply 'regulated' during manufacture to shoot the factory ammunition of the day into a composite group, as described above.

Our challenge today, now that original factory ammunition is no longer available, and neither cordite powder nor in some cases correct bullet weight or shape exist, is to try to assemble handloads that emulate the original ammo well enough to 'regulate' in the old double rifles.

Hope this explains it....