As far as I am aware, there is nothing limiting the location where a C3M is mounted, but other rules question threads have stated that a C3M mounted in a 'Mech uses the front arc and a C3M mounted in the body of a vehicle also uses the front arc.
Can I use different firing arcs by mounting the C3M in other locations? For instance can I mount it in the arm of a 'Mech to use the relevant arm arc or the turret of a vehicle to allow me to aim it 360 degrees with the rest of the turret mounted weapons?