The M26-MASS (Modular Accessory Shotgun System) is a shotgun configured as an underbarrel ancillary weapon attachment mounted onto the handguard of a service rifle, usually the M16/M4 family of United States military, essentially making the host weapon a combination gun.
Insurgency: Sandstorm[]
"Underbarrel magazine-fed pump-action shotgun that uses standard buckshot shells."
— In-game description
The M26 MASS is a underbarrel shotgun attachment featured in Insurgency: Sandstorm. It is a bolt-action shotgun with a detachable magazine. Currently, it is only available to the M16A4, M4A1, and AUG A3.
The M26 is also available as a fully-functional standalone weapon, but only via console commands.