Shotguns in Vietnam

A Remington 870 Express
           In nearly every conflict in American history that soldiers marched off to war, some packed their shotguns. The Shotgun proved itself in the in jungle warfare during WWII when American soldiers involved in savage battles with the Japanese on islands throughout the Pacific used the weapon. After WWII, British soldiers involved in the counter-insurgency battles in Malaya also found shotguns effective. In-country the Shotgun was  used by US Army, Helicopter crewmen, Brown Water Navy, Marines, Air Force, ARVN and even the VC! The weapon was not particularly favored by Australians forces.
         Shotguns are essentially a close range weapon making it ideal for Vietnam. Most encounters in Vietnam and in most guerrilla wars averaged around less than 75 yards.  Your typical shotgun has an effective range of 50 yards. For the lone individual assigned point who slowly moving  along in the boonies pausing occasionally the shotgun  gave him a firepower advantage against in enemy that could appear. One shot would win the day in these situations and it was then that a shotgun that mattered.
        The tunnel rats also put shotguns to good use. Sgt. Flo Riviera was able to get a 4-gauge riot shotgun approved; "real handy that four-gauge, the noise blew your eardrums out but if there was anything at all infront of you, you hit it." With testimony like that it's easy to see why the Tunnel Rats grudgingly nicknamed their shotguns "Cannons." In the tight underground passages the weapon was ALWAYS effective, many a NLF soldier learned this the hard way.
        Outside the jungle, shotguns were also effective for perimeter protection at airstrips, FSBs,  and other strategic locations. Here the shotgun's lack of range was not a problem. Charging NLF/PAVN proved an easy target for buckshot. At night the shotgun's wide spread also helped "find" enemy who were hidden. Shotguns were also came in handy in the infrequently occurring urban battles of the war.
        Shotgun's received a mixed response from the ARVN. The stout ARVN soldier found the shotgun to be rather cumbersome and to large to sustain a high rate of fire. The NLF scavenged and stole everything from empty C-ration cans to entire tanks during the war. Thus, they eventually acquired a few shotguns. A double-barreled shotgun encountered in 1968 had several few modifications. It was sawed-off and had been fitted with the "paratrooper-style" folding stock of a US M1 Carbine. The VC also manufactured crude shotguns out of sections of pipe.
        A wide variety of Shotguns were used in Vietnam by US forces, due to the large number of shotguns grunts would have shipped to them from home. A shotgun from home gave a grunt an unparalleled familiarity with his weapon and thus a tactical edge. The most common shotgun was Remington 870; a 12-gauge pump action weapon, still in production today. It got off to a rocky start in Vietnam. The shells initially issued contained 8 "00 buck" pellets. A weapon with #4 buck (41 pellets) would've been more useful. Later as things improved a special adapter was added to help focus the spray of pellets. Other shotguns used included the Winchester Model 12, Steven's 12-gauge and the Ithaca Mod. 37 all pump actions.
        Many US weapon systems copied the success of the shotgun. With either pellet or flechette rounds. The 90mm gun on a M48 Patton the standard anti-infantry weapon was "canister" and on the M551"shotshells." The M79 Grenade Launcher received a flechette/pellet  round so did the Grenade Launcher attachment of the M16. The Limited Warfare Laboratory even came up with a shotshell round for  the tunnel rat's .44 Magnum! But; to the US forces who used a shotgun their was really no substitute.
 
Sources: Weapons: An International Encycopedia From 5000 BC to 2000 AD, The Tunnels of Cu Chi, Fire & Movement(1967)
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US COMBAT SHOTGUNS(book)