M26 Modular Accessory Shotgun System (MASS)...
http://peosoldier.army.mil/factsheets/SW_IW_M26.pdfSpecificationsCaliber: 12 gauge
Operation: Straight pull bolt-action.
Capacity: 5 round detachable magazine.
Ammunition: 2.75 and 3 in lethal, less-lethal and breaching rounds.
Barrel length: 7.75 in (197 mm) with integral breaching stand-off adapter.
Under-barrel version:
Overall length: 16.5 in (419 mm)
Weight: 2 lb 11 oz (1.22 kg)
http://Stand-alone version:
Overall length: 24 in (610 mm) (stock collapsed)
Weight: 4 lb 3 oz (1.90 kg)