Ordnance Notes -- by Bob Stoner GMCM (SW) Ret.
106MM Recoilless Rifle with M8C Spotting Rifle
A recoilless rifle or RCL has a rifled barrel, unlike the smoothbore rocket launchers of the WW2 era. That is, the 2.36-inch rocket launchers M1 and M9 used fin-stabilized, solid propellant rockets. What differs with the RCL is its ammunition looks like a conventional round of fixed ammunition, but with the case resembling a Swiss cheese with all its vent holes.
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 makes it a huge propellant hog.) It differs from the rocket launcher in that it fires a shell (with a 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 launcher 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 for from the M1917A1 or M74 machinegun tripods. The 75mm RCL M20 was fired from the M1917A1 or M74 machinegun tripods. The 90mm RCL M67 was fired from the shoulder. 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.
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 M8C was designed to be the primary ranging device for the weapon. The .50 caliber spotter round was much shorter than the .50 caliber Browning machine gun cartridge. The spotter round was a ballistic match to the HEAT and HEP rounds fired by the M40A1. On impact with the armored target, the gunner saw a puff of white smoke in his sight. If the puff of smoke was on target, he fired the main gun. If not, he made his traverse and elevation corrections and fired the .50 spotter again. A one-shot hit with the 106mm was imperative, because the back blast from the breech extended in a cone-shaped fan 75 yards deep and 150 yards wide. A tank would have to be both stupid and blind to allow the 106mm gunner a second shot under such 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 of the projectile crumples until the cone comes in 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 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. The projectile of the HEP round is more streamlined and rounded than the conical-shaped nose of the HEAT round. The HEP round operates against 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 plate. Milliseconds after the hit, a base detonating fuze explodes the filler. The explosion sends a shock wave through the armor plate that breaks off a huge chunk on the opposite side of the plate and sends it bouncing around the inside of the tank at very high velocity. Armor hit by HEP exhibits a characteristic circular scorch mark on the outside and a huge crater (where the armor was blown away) on the inside.
APERS round. The APERS round converts 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 at 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 training.
The M40A1 is a crew-served weapon; that is, more than one man is required to operate it efficiently. However, one man can also do it at a reduced rate of fire. The following procedure assumes that both the M40A1 and M8C have been bore sighted to the main optical sight.
The loader pulls down the breech handle and swings open the breech block. The loader inserts the shell and indexes its 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 traverse wheel mounted horizontally on the tripod base. For fast moving targets he can declutch 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 semi-automatic and fires from a 10- or 20-round magazine.) When the button is pushed IN the 106mm round is fired.
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 he is on target.
The weight of the complete M40A1 weapons system is approximately 485 pounds. It has an overall length of 11.2 feet and a maximum range of 8,420 yards. Muzzle velocity of the round is approximately 1,650 feet per second. Armor penetration is approximately 6 inches at 60 degrees to the vertical.
© 2005 Bob Stoner R3