The forcing cone is at the start of the barrel. The cylinder will be misaligned to the barrel to a very small extent within manufacturers tolerances. The cone forces the ball to a better position.

In a cartridge firing gun the bullet sits right at the start of the rifling. It does not go straight forwards but takes up the helical motion from that start.

With m/l revolvers there is no rifling in the cylinder nor in the forcing cone. The ball starts moving forwards without twisting for over half an inch, then we are asking it to change state to a helical movement.

Accuracy can be improved by keeping that distance as short as possible. Rather than seating the ball a long way into the chamber it should be as close as possible to the barrel.