The M26, a 12-Gauge Modular Accessory Shotgun System (MASS), is under
development. It is intended for attachment underneath the M4 Modular Weapon
System (MWS). It zeroes to the host weapon and fires lethal, non-lethal, and door
breaching rounds.
The M26 is also designed as a stand-alone system and comes with a collapsible
buttstock. With a Picatinny rail on top, the receiver can be used to mount sighting
equipment.
The bolt handle is mountable on either side for ambidextrous handling.
Source:
http://peosoldier.army.mil/factsheets/SW_IW_M26.pdf