Don't know much about chest rigs personally, but this may help:
You can keep 6 or 8 or whatever shells in this velcro holder, and keep it attached to your chest rig. I've heard they will fit into AR-mag sized pockets. S&J does offer a pouch of their own for it, too:
I've got the detachable shell carrier for my 870 and I love it. In a SHTF scenario, it would be extremely valuable having those extra shells handy.