OK, let's go step by step:
1) Barrels are actually convergent on a double. This is a fact.
Allow me to crudely demonstrate, using my .470 Krieghoff:
A cursory measurement shows that the center to center distance between the two firing pin is 0.95something" (we do not need minute precision here), hence the center to center distance between the two barrels at the cartridge head is 0.95something"...
A cursory measurement shows that the distance between the two barrels bores is 0.33something" (again, we do not need minute precision here). Since the diameter of a .470 bullet is .474, the center to center distance between the two barrels at the mouth is .33something" + .474" = is 0.80something"...
Mathematically, the two barrels converge from 0.95something" center to center at the cartridge head - 0.80something" at the barrel mouth = 0.15something" convergence over the length of the barrels.
That is a fact.
2) Regulation
Because the barrels factually converge, if the convergence of the barrels was the only factor, by definition the barrels would cross at one point down range.
But convergence is not a by-product of barrels assembly (barrels could just as well be assembled to be parallel or to diverge). Convergence is the by-product of regulation. Regulation is designed to compensate for the primary characteristic of side by side double rifles recoil (beside the barrels going up), which is the yaw.
In a side by side double, when the right barrel is fired, since it is located on the right side of the axis of the stock, the rifle twists to the right as it starts climbing under the recoil. As a consequence the right barrel muzzle points upward and to the right when the bullet leaves the pipe. The same happens in reverse with the left barrel.
The purpose of regulation is to get the barrels to converge just enough so that under yaw the barrels are pointed straight when the bullets leave the tubes. This is why
Red Leg says:
a properly regulated rifle ... fires parallel until the bullets hit dirt.
Notice that the rifle fires parallel because the convergence of the barrels negates the divergence from the sighting plane caused by the side-way yaw of each barrel in its direction under recoil.
This dynamic is what makes regulating barrels so difficult because the convergence is generally fixed on the large caliber rifles (although in some rifles it can be adjusted as pointed out by
HWL) but the recoil and recoil management are not fixed. Factors at play include:
- bullet weight and bearing surface: this influences how long the bullet will take to exit the barrel, hence where the barrel mouth will be under recoil when the bullet leaves;
- propellant charge (amount, burn rate, etc. - including from ammo lot to ammo lot): this too influences how long the bullet will take to exit the barrel, hence where the barrel mouth will be under recoil when the bullet leaves;
- whether the rifle can physically yaw: off hand shooting will produce different results than shooting from an over-tight bench rest set up (God forbid! a lead sled LOL !!!);
- physical characteristics of the shooter and ability to control recoil: a 150 lbs. shooter will not have the barrels in the same spot under recoil, as a 250 lbs. shooter will when the bullet leaves; a shooter rolling with recoil will have different results from a shooter fighting recoil; etc. etc.
This is why in the old days the Brits used to ask to finalize the regulation of the barrels with the actual client actually shooting the gun, from a standing position with a simple shelf rest only bearing the weight of the rifle to steady the aim and allowing an unhindered recoil and yaw.
This is why too, the vast majority of mass produced European doubles are over & under instead of side x side, because there is no lateral yaw with an over & under (both barrels are in the axis of the stock) and they are much easier to regulate for vertical dispersion than the side x side are for horizontal dispersion.
So, factually, a double that crosses can indeed be of poor manufacturing or shoddy regulation (and there has been no shortage of such from manufacturers that will mercifully remain unnamed; suffice it to say that one generally get their money's worth ;-) but this may also be a byproduct of the rifle being shot with a different load than the one it was regulated with (which is why makers always supply - or should always supply - a regulation target that identifies the load used to regulate); and this may also often be the byproduct of a shooter new to big guns and not controlling recoil properly, which can be corrected with practice.
Once the barrels are
regulated, then the rifle may be
sighted, which generally involves mounting the proper front sight height (and occasionally changing the rear sight height), and drifting sideways the rear sight, until the regulated group impact in elevation and direction the point of aim. Obviously, a regulation that would locate a group 1 foot high and two feet left would be defective, and proper regulation must be achieved within a couple inches of target center before it is worth adjusting the sights for final sighting.
A well
regulated double will shoot the right barrel on the right side, and the left barrel on the left side, separated by no more than 2" (1" is the holly Grail) from muzzle into eternity. This is when they truly
shoot parallel. When properly
sighted this rifle will place these two shots at point of aim at 50, 75, 100 yd or whatever distance you want, based on the height of your front sight compensating for the rainbow trajectory of big Nitro Express cartridges.
I hope this helps
. Thank You!
