The CA MK16 Enlarge Stock is designed to replace the standard stock of the Classic Army FN MK16. Although this replacement stock does not retract it has more room for a larger battery pack like the custom 10.8v 1700 NICD battery designed for this stock. A larger battery pack should give you more hours of game play and longer periods in between troublesome battery exchanges.
Buy Sell Trade info here. Possible future feature.