There's lots of ways to deal with this, the simplest being a simple few second hold time for instant convergence after you put your cross hair on a mech(not target the mech but simple hold the cross hair on a point). I am NOT suggesting a cone of fire or random spread. Simply have the weapons dial in from a neutral, straight forward, default position given their distances apart on a mech to a perfect convergence over a few seconds.
Let single fire weapons always hit the cross hair. Allow actuators in arms to negate the need for dial in time for arms will full range of motion... so if you've got a mech will full arm actuators on both arms the arm weapons will hit center without the dial in. (this gives the low slung slot poor arms on full actuator mechs a tangible benefit over high mounted arm pods with no swing)
There are lots of more complicated schemes floating around, this is far and away the simplest I've read.
Edited by Prezimonto, 15 August 2013 - 08:20 AM.