The dense jungle environment in Vietnam often meant that fighting was at close quarters. In these circumstances, the 12-gauge shotgun's short-range firepower and wide shot spread were highly prized by some U.S. soldiers. In addition to Army Infantry squads, shotguns were also used by Navy SEALs and Riverine forces operating in the Mekong Delta. The Viet Cong constructed their own 12, 16 and 20-gauge shotguns from water-piping!1 Outside the jungle, shotguns were carried by some soldiers and MP's whilst patrolling base areas.
1. These homemade weapons boasted an effective range of 25 to 30 meters.
Weapons And Material In The Hands Of, Or Possibly Available To, The Viet Cong - First Edition. (MACV 1964)