Well regulated double barrel rifles are regulated to shoot parallel trajectories, the mechanical convergence of the barrels being regulated to overcome the diverging yaw of the SxS rifle under recoil. The fact that indeed may double rifles cross, is not by regulation design, it is either as a result of poor regulation, or shooting a rifles with a load different from the regulation load. The reason for this is that the characteristics of the regulation load (bullet weight, bearing surface, powder charge, burn rate, etc.) all combine for the bullet to exit the barrel at the precise moment where the trajectories will be parallel. This is called "barrel timing." Bullets that exit the barrels before this precise point of the recoil yaw because they are lighter, faster, have less bearing surfaces, etc. will cross. Bullets that exit the barrels after this precise point of the recoil yaw because they are heavier, slower, have more bearing surfaces, etc. will spread. The convergence of the barrel is based on the regulation load. Other loads can be developed with other bullets (lighter, heavier, different constructions, etc.) but the load must be developed so that perfect barrel timing is achieved. Alternatively some people prefer to have their rifles re-regulated to a new load. This is achieved by de-soldering the barrels, moving the wedge that regulates the convergence of the barrels, and re-soldering the barrels. A well regulated double will shoot good groups from muzzle to well passed 100 yards, some out to 200 yards because perfect parallel trajectories have been achieved.
