One of my favorite crew served weapons that I played with in the Brown Water Navy in Vietnam was the M40A1 106mm recoilless rifle or RCL.
We used the M40A1 for two major purposes: blowing bunkers with its anti-tank rounds and guarding our boat's flanks with its APERS [anti-personnel] round. Here's the straight dope on recoilless rifles and rocket launchers.
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The 106 millimeter recoilless rifle and its M8C .50 caliber spotting rifle are now largely withdrawn from active and reserve service within the U.S. armed forces. The M40 is the largets in a long line of recoilless rifles developed at the end of World War 2.
ABOVE: The M18A1 57mm recoilless rifle in action against North Koreans during the Korean War (1950-1953). The M18-series saw action in both European and Pacific battles in late 1944 and 1945. The M18 could be fired from the shoulder as shown, or with the aid of its built-in folding bipod, fired from the prone position or from the M1917A1 or M74 tripods. U.S. Navy SEALs and Army Special Forces used limited numbers of M18 recoilless rifles in Vietnam fighting. (Photo: U.S. Army)
BELOW: The M20 75mm recoilless rifles was the bigger brother of the 57mm M18 recoilless rifle. The M20 first saw action during the Okinawa Campaign in 1945. The M20 was always fired from either the M1917A1 or M74 tripods as shown. Note the perforated shell case and pre-engraved rotating band that immediately identify a recoilless rifle cartridge. (Photo: U.S. Army)
A recoilless rifle or RCL has a rifled barrel, unlike the smoothbore rocket launchers of the World War 2 era. That is, the 2.36-inch rocket launchers M1 and M9 that used fin-stabilized, solid propellant rockets. What differs with the RCL is its ammuniton that resembles a conventional round of fixed ammunition, but with the case that looks like a Swiss cheese with all its vent holes.
The M9A1 2.36-inch (60-millimeter) rocket launcher and its rocket. Compare this with the ammunition for the 75mm recoilless rifle. Satilization of the rocket in flight is done by the fins. Stabilization of the recoilless rifle projectile in flight is done by the spin imparted by the pre-engraved rifiling band and the rifling of its barrel. (Photo: U.S. Army)
Like the rocket launcher, the recoilless rifle can be lightly built due to its method of operation. The RCL vents an equal mass of propellant gases to its rear as its projectile exits its front. (This feature makes it a huge propellant hog.) It differs from the rocket launcher in that it fires a shell (with pre-engraved rifling band) instead of a spin-stabilized rocket. Holes are formed in the case of the recoilless rifle cartridge. The holes allow the propellant gases to escape and exit to the atmosphere through vents in the rifle's breech block. When the mass of the gases being exhausted to the atmosphere equal the mass of the projectile being fired, the laucher or rifle is "recoilless."
Depending on its size and caliber, the recoilless rifle may be fired from the shoulder or from a mounting. The 57mm RCL M18 or M18A1 could be fired from the shoulder or from the M1917A1 or M74 machinegun tripods. The 75mm RCL M20 was fired only from either of the same tripods. The Vietnam-era 90mm RCL M67 was fired from the shoulder or prone positions. The 105mm RCL M27 was fired from a permanently-attached vehicular mounting. The 106mm RCL M40A1 was fired from an attached mono-wheeled M79 tripod or a vehicle.
The 106mm RCL was designed as a light weight anti-tank weapon that could kill main battle tanks. Ammunition was high explosive anti-tank (HEAT), high explosive plastic (HEP), anti-personnel (APERS), and drill (inert). The .50 caliber spotting rifle M8 or M8C was designed to be the primary ranging device for the weapon. The .50 caliber spotter cartridge was much shorter (12.7x76mm versus 12.7x99mm) than the standard .50 Browning machine gun cartridge. The spotter cartridge was a ballistic match to the HEAT or HEP rounds fired by the M40A1 RCL. On impact with the armored target, the gunner saw a white puff of smoke through his optical sight. If he was on-target, he fired the main gun. If not, he adjusted his fire until the smoke puff was on target. A first-shot hit with the 106mm round was essential when attempting to kill a main battle tank. The back blast from the breech of the 106mm RCL was a cone-shaped fan 75 yards deep and 150 yards wide. A tank commander would have to be either blind, deaf, or stupid to allow the RCL gunner a second shot under these conditions.
HEAT round. The HEAT round uses the Monroe effect to penetrate armor. The cone-shaped nose of the HEAT round is hollow. The hollow inside of the round is roughly diamond-shaped, the upper half of the diamond is the sheet metal nose and the bottom is a copper cone. The empty space in the round forms what is known as the "standoff distance" and the copper cone focuses the blast which punches a hole through the armor plate. The explosive mixture is located in the body of the shell behind the copper cone and is capped with a base-detonating fuze. When the HEAT round hits armor, the nose crumples until the cone makes contact with the armor. Milliseconds after the hit, the base detonating fuze fires to detonate the explosive. The copper cone focuses the energy of the blast on a small spot of armor and burns a hole in it; the copper cone is formed into a slug which is blown through the hole and into the tank. Armor hit by the HEAT round shows a characteristic circular scorch mark with a hole in the center similar to that caused by an acetylene torch.
HEP round. This projectile of the HEP round is more streamlined and rounded than the conical-shaped nose of the HEAT round. The HEP round operates against the armor in a less spectacular, but most efficient way. The HEP shell (called "Squash Head" by the British) is actually a mesh bag filled with explosive inside a thin-skinned projectile. When the HEP shell hits the armor, the projectile skin breaks away and the explosive-filled mesh bag mushrooms out against the armor plate. Milliseconds after the hit, a base detonating fuze explodes the filler. The explosion sends a shock wave through the armor plate that breaks of a huge chunk on the opposite side of the plate and sends it bouncing around the inside of the tank a very high velocity. Armor hit by HEP exhibits a characteristic circular scorch mark on the outside and huge crater (where the armor was blown away) on the inside.
APERS round. The APERS round onverts the RCL into a giant shotgun for use against infantry. The projectile is filled with 6,000 13-grain flechettes (looking like nails with fins) stacked nose-to-tail. The APERS round resembles the HEP round, but it has a nose fuze that allows it to function at the muzzle or a a preset distance from the muzzle of the RCL. The APERS round also has a tracer element in the base of the projectile.
DRILL round. The DRILL round is an inert cartridge that simulates the size and weight of the HEP round. It is used for crew training.
The M40A1 is a crew served weapon that requires two men to operate efficiently. However, one man can do both jobs of loading and firing in emergencies. The following procedure assumes that both the M40A1 and M8C have been bore-sighted and aligned with the optical sight.
The loader pulls down the breech handle and swings open the breech block. The loader inserts the shell and indexes the pre-engraved rifling with the rifling in the gun barrel. The loader closes the breech block and positions himself forward and to the right of the breech.
The gunner sits on the tripod leg and acquires the target through his optical sight. He tracks the target using a graverse wheel mounted horizontally on the tripod base. For fast moving targets, the gunner can de-clutch the traverse wheel and move the gun with his body. He adjusts elevation by a vertical hand wheel on the gun mount. There is a mushroom-shaped button in the center of the elevation wheel. When it is pulled OUT, the .50 caliber spotting rifle fires. (The .50 caliber spotting rifle is a gas-operated, semi-automatic rifle and fires from a 10 or 20-round magazine.) When the button is pushed IN, the 106mm round is fired.
ABOVE: A left side view of the M40A1 106mm recoilless rifle (RCL) and its M8C .50 caliber spotting rifle. The prominent hand wheel below the spotting rifle's rear barrel clamp is for elevation adjustments. The mushroom-shaped buttom in the middle of the had wheel is the firing button for the .50 spotting rifle M8C (OUT to fire) and M40A1 (IN to fire) 106mm rifle. The traversing hand wheel is on top of the gun cradle wehre it joins the M79 tripod (to the rear of the mono-wheel).
The gunner uses the hand wheels to put the gun on target. He ranges the target with the .50 spotter rifle whose ballistics are the same as the 106mm round. He follows the .50 spotter round to the target by following its tracer trail and notes the location of the white smoke puff. If on target, he fires the main gun. If not, he continues ranging until it is on target.
Weight of the complete M40A1 weapons system is approximately 485 pounds. It has an overall length of 11.2 feed and a maximum range of 8,420 yards. Muzzle velocity of the round is approximately 1,650 feet per seconds. Armor penetration is approximately 6 inches at 60 degrees to the vertical.
ABOVE: The Heavy SEAL Support Craft (HSSC) in late 1969 wiht its bow ramp down. The gray Mk 2 Mod 0 Navy 81mm mortar is shown with its muzzle down. Directly above it is the 7.62mm Mini-gun (pointing at the camera). To its rear is the M40A1 RCL with its breech open. The .50 caliber Browning machine guns point from beneath the cover over the well deck. The diamond-shaped plates next to the .50's are ceramic armor plates to protect the gunners from side shots.
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