7" LaRue rail. LT606-1 mount on the left rail, shot with a VFG and 'high thumb' this illuminates the target without getting much barrel shadow, and on a short rail also illuminates the front sight. Light is a G-3. Same mount and light I used on active duty on my issued M-4 in the same location. Having tried the bottom, right and top rails at various times, due to carrying an M203, quadrant and leaf sights, Aimpoint and PEQ2 the left rail was all that was left open, and ended up working well enough I stuck with it.
In the event you switch shoulders you can thumb-over-rail and still activate the light. Not perfect but it works.