Tunnel Weapon: The Bang in the Dark

By Kevin Dockery

During the Vietnam War, the United States fought the Viet Cong and North Vietnamese Army forces with some of the most high-tech weapons the US could produce. These weapons were used in against some of the most low-tech forces the US military had faced in a long time. The arena of combat ranged from swamps to mountain highlands, from the water to the land. It even extended from the air above North Vietnam to the ground underneath South Vietnam.

One of the first major US search-and-destroy operations, where troops went out and tried to hunt down and exterminate the VC in their strongholds northwest of Saigon, was called OPERATION CRIMP. Beginning in early January, 1966, US and allied troops were almost immediately ambushed and fired on by an enemy who disappeared as suddenly as he had first made himself known. The means of this military magic was discovered by a US Army sergeant when he sat on a nail that was sticking up from a hidden trapdoor.

The sergeant’s painful discovery was the first uncovering of the tunnel complexes of the Viet Cong. Tunnels dug into the root-sewn red clay soil could extend only a short distance, or be a part of huge, multilevel, underground complexes that would extend for miles. There was only one way to find out just what was in the tunnels - a soldier had to go down into the dark and search them.

Army dogs didn’t work to search the tunnels, they were killed by unseen traps too quickly. So a new type of American soldier had to come out from the ranks to face the VC in the new arena. Tunnel Rat was the term that was quickly coined for these generally small-stature soldiers who would go down the holes. Like green ferrets, Tunnel Rats would go into the dark, forbidding tunnels and discover whatever might be there.

Lurking in the warren of dark, hot, humid, tunnels, the tunnel rats might find boobytraps, snakes, venomous insects, weapons caches, or a weapon in the hands of a very live VC. One thing that the tunnel rats were always sure they could find underground - a quick death.

Tunnel entrances were often little more than an 18-inch square camouflaged wooden trapdoor. Through this trapdoor would go a tunnel rat equipped with a flashlight, handgun, knife, and bravery.

The subterranean world of the tunnel rat was made up of tunnels dug through soil that was described by an engineer as being like natural, reinforced concrete. According to the Viet Cong plans the tunnels were no smaller than 0.8 meters or greater than 1.2 meters wide. Ceilings were no greater than 1.8 meters high, and much more often closer to the minimum 0.8 meters high. But the larger tunnels were usually in the deeper layers of the complex. The areas usually searched by the tunnel rats were often little more than 30-inch wide tubes cut through the sticky earth.

The M1911A1 Colt .45 automatic was the general handgun used by the tunnel rats during their early explorations. But the sound of the big .45 going off in the close confines of a tunnel was deafening. The M1911A1 was also considered too big and too cumbersome as well as being too loud.

Revolvers were chosen by a number of tunnel rats as their search techniques became more sophisticated through experience. The .38 revolver was relatively small, light, and easy to handle. In addition, the Smith and Wesson or Colt weapons available could handle the dirt and muck of the tunnel environment easily, and a second shot in case of a misfire was only a trigger pull away.

In the condensed world of the tunnel rat, even the M2 carbine with a folding stock was nicknamed “The Cannon.” IN general, pistol suppressors were not available, and when they were, the tunnel rats found them to be long and made their handguns ungainly.

A tunnel exploration kit was developed in 1966 by the Limited Warfare Laboratory (LWL). The communications system in the kit didn’t work well. The cap-mounted lamp slid around and either was pointing the wrong way, slipped down over the tunnel rat’s eyes, or gave the VC an aiming point that was almost guaranteed to be fatal.

The weapon in the kit was a .38 Special Smith & Wesson Model 10 (M&P) revolver with a 4-inch barrel, a small high-intensity aiming light and a muzzle mounted suppressor. The hip-holster issued with the weapon was very hard to draw from in the tight confines of a tunnel. The revolver was huge with its aiming light and muzzle can. And even with a tight cylinder gap, the suppressor didn’t reduce the sound of the shot enough to be worth the trouble.

The designers back at the LWL probably hadn’t had a lot of faith in the revolver suppressor as well, they issued ear plugs in the kit to be used when firing underground. The Tunnel Exploration Kit was abandoned in service, most tunnel rats wanted nothing to do with it, and it was quickly withdrawn.

By 1969, subterranean combat in the tunnels of South Vietnam had been refined by time and experience. The tunnel rats knew what worked for them and were exceedingly conservative in adopting any new hardware or weapons. It was in July 1969 that ten of possibly the rarest modern US military handgun ever made arrived in Vietnam.

Earlier, in December, 1967, the U.S. Military Assistance Command, Vietnam (MAC-V) had recognized that the need still existed for a low-noise weapon for what were now officially called the tunnel exploration personnel in the Republic of Vietnam. The tunnel rats had been using a variety of weapons since the suppressed S&W fiasco eighteen months earlier. Some tunnel rats had competent armorers and machinists make up suppressors for a variety of handguns, including several for the M1911A1. But the big .45 didn’t operate dependably with a simple muzzle can, though any sound suppression given the big pistol was well liked by the rats who didn’t mind its size.

A few of the OSS-ordered WWII era suppressed High Standard HD .22 automatics made they way into Tunnel Rat hands. But these weapons were very few in number and wanted by a number of other special units. Personal weapons were used by the rats, ranging from .25 caliber automatics to sawed-off shotguns. MAC-V looked at some of these field improvisations and added their characteristics to their request for a new weapon.

To increase hit probability in the dark tunnels, ammunition for the new weapons would be of the multiple-projectile type. A spread of shot would be much easier with when the target may only be visible for a fraction of a second, or maybe not visible at all. Since the tunnel combat was by nature at a short range, the weapon was to be lethal at 25 feet, minimum. Basically, MAC-V wanted a small, handheld, suppressed shotgun for the tunnel rats.

The Army LWL passed on the project to a qualified civilian contractor, Aircraft Armaments Inc. (AAI) of Baltimore, Maryland. AAI had been working for a number of years on various applications of captured piston and expanding capsule (teleshot) technology for both the military as well as NASA and the aerospace industry. The company’s experience with firearms had them quickly applying the captured piston concept to the tunnel weapon project.

In the captured piston system, propellant gases from normal smokeless powder drives a piston pushing a payload down a cylinder with one open end. When the gas-driven piston strikes a constriction at the open end of the cylinder, it is trapped and held in place. The payload continues away from the cylinder, moving at what can be a significant velocity.

The captured piston seals off the end of the cylinder, trapping the propellant gases inside the system where they can bleed off slowly. With no propellant gases escaping violently, muzzle blast, flash, and the resulting noise, is reduced considerably. Exchange a steel cartridge case for the open-ended cylinder, and a plastic capsule full of shot for the payload, and you have the heart of the tunnel weapon.

The “muzzle velocity” of the projectile, in this case the shot charge, is dependent only on the length of travel and pressure pushing the internal piston of the round. A sealed-in percussion primer ignites the propellant charge in a normal way. Heavy threads at the muzzle end of the cartridge slow and stop the piston as it shears through them. The capsule continues out the barrel of the weapon and breaks up on hitting the resistance of the air, releasing its shot charge.

To increase the lethality of the shot charge, heavy metal shot was used instead of the usual lead shot. Sintered tungsten-steel alloy shot was formed under pressure for the subprojectiles. The sintered-metal technology involved compressing powdered alloy in a die between two punches and gave the shot a noticeable belt in its center, topped by two hemispherical ends. The shot looked much like a round ball with a band around the center.

To fire the new ammunition, a revolver system was used to eliminate any problems with trying to function a semiautomatic weapon. To maximize available space for the ammunition design, the largest commercially-available revolver design was chosen for development into the tunnel weapon. The revolver decided on was the Smith & Wesson Model 29, .44 Magnum.

The .44 magnum barrel was removed and a simple short cylinder threaded into its place. The stub barrel had a 0.400 smooth bore reamed through its center. The extractor rod of the revolver was cut back to just in front of the end of the muzzle. Outside of the barrel, the main mechanism of the revolver was little changed outside of the cylinder.

Each chamber of the .44 magnum Model 29 cylinder was reamed out to 0.528 inches straight for its entire length. Over 2.75 ounces of steel were removed from the cylinders that originally weighed over half a pound. This gave the cylinder walls of the tunnel weapon enough strength to function and carry the ammunition, and that was about all. But since the steel cartridge case itself held the pressure of the propellant gases, the cylinder walls could safely be made very thin.

An additional coil spring was placed near the top of the mainspring to give the hammer additional power to hit the slightly recessed primers of the special ammunition. The precision sights of the Model 29 were useless with the new ammunition and the rear sight was removed, leaving a square slot in the top of the revolver for rough aiming if desired.

When fired, the new ammunition gave out little more than low-intensity sparks. The blast and noise of a normal revolver shot was almost completely eliminated. The 15-pellet load gave the revolver the shot spread of a very open .410 shotgun.

The ammunition did have an unusual safety problem inherent in its design. The round didn’t have to be in a barrel or cylinder to have a lethal velocity if fired. In case of a round going off from heat or a blow, the shot would be moving at the same 750 foot per second velocity it would have if fired from the weapon. To insure safety in storage, special 1/8 inch thick steel liners were put in the ammunition cans holding the tunnel weapon ammunition. If the rounds went off in a fire or other mishap, at least the pellets wouldn’t leave the ammunition can.

The shot from what was now called the Quiet, Special Purpose Revolver, would penetrate a 3/4 inch thick sheet of plywood at 15 feet. This was later used as part of the demonstration of the weapon to its users in Vietnam. In July, 1969, ten Tunnel Weapons and 992 rounds of ammunition arrived in Vietnam for testing.

Five of the new weapons and 496 rounds of ammunition were each issued to the 1st and 25th Infantry Divisions for a 90-day field test period. On 22 August, 5 Tunnel Weapons and 125 rounds of ammunition were transferred from the 1st Infantry Division to the Americal Division for the balance of the testing period. Training time for the new weapons ranged from none to several hours, but it usually always included a firing demonstration at the recommended plywood target.

Accuracy was reasonable with the new weapons. One user could hit a flying sparrow at 20 meters with a single shot. But that marksman was very much the exception. Most users trained for an hour or less with the new weapons, firing 30 or fewer rounds each during the session.

Acceptance of the new weapon was limited by the very conservative tunnel rats. But within weeks of their arrival in Vietnam, the tunnel weapons were being carried in their special shoulder holsters into the tunnels of Vietnam.

The “silent” quality of the tunnel weapon gave some of the users questions regarding its lethality. During one tunnel mission, a sergeant with the 25th Division demonstrated the effect the tunnel weapon had on human targets.

While slipping through a tunnel complex, the sergeant crawled around a corner and met face to face with an NVA soldier; “The NVA was sitting and just as surprised as me, but I was able to fire the Tunnel Weapon.... before he could use his rifle. I drug him outside but he was dead.”

In another incident, a sergeant armed with a Tunnel Weapon took out a VC outside of a tunnel. “I shot him coming out of a bunker. He was running, and he was about 20 to 25 feet away.”

The shot load of the Tunnel Weapon was effective but it didn’t have the stopping power of a regular bullet. A number of other encounters with VC and NVA troops in tunnels resulted in wounded and disabled enemy forces, but they were not killed by the shot load of the Tunnel Weapon.

The steel cartridge cases also had some problems with the recessed primer system. The steel firing pin of the revolver didn’t always strike the primer solidly enough for it to go off. This resulted in a high (18 percent) failure to fire rate on the first weapons. The firing pin was replaced in-country with a harder metal one, and the ammunition primer redesigned. But the very conservative tunnel rats never liked or trusted the tunnel weapon completely. What they had been using for several years had worked for them and they were still alive. That was enough.

But another group of US forces liked the tunnel weapon - a lot. The 25th Infantry Division issued their Tunnel Weapons also to members of one of their reconnaissance and Intelligence platoons as well as to its Ranger company and infantry companies. The Americal Division used its Tunnel Weapons with its Ranger and Infantry companies. This put the tunnel weapons to use in ambushes, prisoner snatches, and other more covert actions.

The Tunnel Weapon was found to be ideal for ambushes, especially at night. Enemy soldiers were killed at ranges of less than ten feet with other troops nearby never hearing the shots. Comments on the weapons from these users included;

“If we had five such pistols we could stay out for a week at a time...without ever giving our position away.”

“That pistol is the most magnificent weapon I ever seen in action. I could use three of these in my platoon. The pistol doesn’t make any noise louder than a cap pistol. It hits the target just by using the pointing method. I never aimed the weapon. This weapon is far by being better than a .38 pistol we had. It’s more accurate, it’s very quiet, and it handles better than any other pistol I’ve handled.”

“For our use it works real good. We have to avoid contact and the weapon doesn’t compromise our position. For our platoon we could use five or six of these weapons.”

There were some suggestions made for improving the weapon, and especially the ammunition. The primer pocket was redesigned and that eliminated a lot of the misfire problems. But besides its use in 1969, the Tunnel Weapon was never fielded again in any numbers after the evaluation period. Only the ten examples, and a single specimen maintained by AAI, were ever made. Two of these are at the US Ordnance Museum at Aberdeen Proving Grounds, Specimen #5 is on public display there and the AAI specimen is there in storage.

On a very odd note, one of the Tunnel Weapons was reported lost in Vietnam. This weapon was later recovered in California where it had been used as a suppressed weapon in a murder. Almost no live specimens of the ammunition exists, especially not in private hands. The captured-piston ammunition for the Tunnel Weapon and other systems has been declared a suppressor by BATF.

OVERALL LENGTH - 6.75 inches
BARREL LENGTH - 1.372 inches
BORE DIAMETER - 0.400 inches
WEIGHT (EMPTY) - 2.01 pounds
WEIGHT (LOADED) - 2.37 pounds
WEIGHT (CYLINDER) - 0.419 pounds

CASE DIAMETER - 0.526 inch
RIM DIAMETER - 0.550 inch
ROUND LENGTH - 1.866 inches
ROUND WEIGHT - 0.971 ounces (27.5 grams)
PROJECTILE - 3 piece plastic sabot containing 15 tungsten-steel alloy shot
    PELLET DIAMETER - 0.147 inch (belt)
    PELLET WEIGHT - 7.5 grains (each)
MAXIMUM RANGE - 1,400 feet
SOUND LEVEL - 120 Decibels at 1 meter from muzzle

This article first appeared in Small Arms Review V5N9 (June 2002)
and was posted online on February 14, 2014


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