Cutaway model of a 57 mm M307 HEAT cartridge for the M18 Recoilless rifle. Development started when captured German 105mm Recoilless guns were investigated. The M18 was developed by Kroger and Musser -two freelance army engineers- during WW2, and used in WW2 and the Korea war, after which the weapon was phased out.
In contrast to German recoilless guns that used a Bakelite blow out base plug in the shellcase - locked up by the rim of the hollow breech-, the M18 used a perforated shellcase which allows the gasses of the propelling charge to move backward through the oversized chamber of the weapon, leaving the weapon through two circular slots segments in the breech.
The Recoilless rifle has some advantages and disadvantages when compared to weapons as the 2,36 bazooka, which was developed in the same time.
The bazooka is very light, however it could only fire a relative light low speed rocket over a limited distance with accuracy diminishing at greater ranges. Therefore it was a short range weapon only.
The Recoilless rifle fired a heavier shell over a greater range with more accuracy, however, weapon and cartridges were considerably heavier than the bazooka. When used for accurate fire over greater distances the M18 recoilless rifle was placed on a tripod that was normally used for the Browning M1917 machine gun.
The biggest advantage of The M18 however was that it allowed the infantry to fire a relatively heavy shell from a light gun. It made the M18 ideal for usage in the jungle, streetfights and any other place where a towed gun could not be easily transported to.
The working principal of the M18 Recoilless rifle can simply be described as weight of gas from breech X speed of gas = weight of projectile X speed of projectile. As long as this is in balance, the weapon is recoilless.
Four types of projectiles were available: The HE M306 (High Explosive ), The HEAT M307 (High Explosive Anti Tank), a WP bursting smoke M308(White Phosphorous), and a canister shot.
The HEAT cartridge was a bit of a disappointment as the penetration was only 63,5mm , against 120mm for the Bazooka. The reason for this is the helmet shape of the cone with a pipe connected which was quite ineffective compared to the pointed straight cone for the Bazooka.
The cartridge depicted in this posting is the HEAT. The projectile body is made of drawn steel. A pre-engraved driving band is placed around the body of the projectile. The red copper shaped charge cone is crimped in place in a rim on the inside of the projectile body. A small steel pipe is placed below the cone, ending just above the booster charge pellet at the base of the explosive charge. The main charge consists of 177 grams pentolite 50/50, placed below the cone and around the pipe.
At the base of the pipe , the tetryl booster pellet is placed, fixated by the base screw. The base screw has a red copper washer between the flange of base screw and the base of the projectile. This to prevent hot powder gasses from reaching the booster pellet upon firing.
The fuze M90 consists of a cast and machined zamac (casting mixture of Zinc Alumium Magnesium and copper) housing, which is screwed over the top of the projectile body. In top, a nose impact detonator (red) is crimped in place. From the nose detonator, a hole runs downward, ending in the S&A mechanism.
The S&A mechanism consists of a brass rotor piece, housing the secondary detonator. In rest, this detonator is placed out of line with the hole in the pipe above, so with an accidental detonation of the nose detonator, the flame cannot reach the secondary detonator in the S&A mechanism. The secondary detonator is fixated in this position by two opposite pins from the centrifugal weight beside it which stick in two opposite holes in the rotor.
The centrifugal weight is a cup with two pins (protruding into the rotor), fixating it. The centrifugal weight has two slots at the opposite side through which a fixating pin is placed. This fixating pin ensures that the centrifugal weight can move outward radially, but cannot rotate like the secondary detonator. A spring inside the centrifugal weight forces the centrifugal weight inward, forcing the two pins into the holes of the rotor, fixating the rotor. Below the S&A mechanism, the main detonator is placed. Upon activation, it will spit a flame down the pipe, which detonates the booster charge in the base.
Upon firing, the centrifugal force will throw outward the centrifugal weight, retracting both pins from the rotor. The rotor will throw outward the heaviest parts , causing rotation and placing the rotor detonator in line with the nose detonator and the main detonator below. Upon impact , the ballistic cap over the nose is crushed, as well as the detonator in the nose, which explodes. The flame travels down the pipe, igniting the detonator in the rotor, which on its turn- ignites the main detonator. The flame of the main detonator travels down the pipe below the cone , igniting the tetryl pellet and the main charge.
The Shellcase M30A1 is a drawn steel shellcase that has been perforated to allow gas escaping . The shellcase is filled with granular powder, ignited by a central tube filled with black powder.
In the neck of the shellcase three notches at 120 degrees each are stamped outward. They are meant as a forward stop and help to centre the shellcase in the forward end of the chamber. Older shellcases have a full brass ring which acts as a forward stop. As the driving belt is pre-engraved the projectile has to be rotated slightly when nearly fully inserted into the chamber until it fits the driving belt into the lands and grooves.
In older shellcases a paper liner on the inner wall of the shellcase keeps the propellant in, as well as dry and clean. Later (post war) shellcases use a polyethene bag as liner.
Length of the cartridge: 47,7cm
Weight of the cartridge: 2,54 kg
Length of the projectile: 19,9cm
Weight of the projectile: 1,25 kg
Length of the shellcase : 303 mm
V0: 365 mtrs/sec
Max range: 4350 mtrs.
Effective firing range : appr. 450 mtrs.
Penetration: 63,5mm steel at all distances. @ 90 degrees
The M18 was used by the following countries:
United states, Taiwan, China (58mm Type 36), North Vietnam (58mm Type 36), Philipines, Netherlands, Tanzania.
It must be noted that the Chinese version is not 57, but intentionally made 58mm calibre. Therefore , Chinese ammo cannot be fired from US manufactured weapons, but the slightly too small US ammo can be fired from the Chinese Recoilless rifle.
An interesting movie, showing weapon and ammunition can be found here:
https://www.youtube.com/watch?v=qsM8EtBZ-EQ
Regards, DJH
In contrast to German recoilless guns that used a Bakelite blow out base plug in the shellcase - locked up by the rim of the hollow breech-, the M18 used a perforated shellcase which allows the gasses of the propelling charge to move backward through the oversized chamber of the weapon, leaving the weapon through two circular slots segments in the breech.
The Recoilless rifle has some advantages and disadvantages when compared to weapons as the 2,36 bazooka, which was developed in the same time.
The bazooka is very light, however it could only fire a relative light low speed rocket over a limited distance with accuracy diminishing at greater ranges. Therefore it was a short range weapon only.
The Recoilless rifle fired a heavier shell over a greater range with more accuracy, however, weapon and cartridges were considerably heavier than the bazooka. When used for accurate fire over greater distances the M18 recoilless rifle was placed on a tripod that was normally used for the Browning M1917 machine gun.
The biggest advantage of The M18 however was that it allowed the infantry to fire a relatively heavy shell from a light gun. It made the M18 ideal for usage in the jungle, streetfights and any other place where a towed gun could not be easily transported to.
The working principal of the M18 Recoilless rifle can simply be described as weight of gas from breech X speed of gas = weight of projectile X speed of projectile. As long as this is in balance, the weapon is recoilless.
Four types of projectiles were available: The HE M306 (High Explosive ), The HEAT M307 (High Explosive Anti Tank), a WP bursting smoke M308(White Phosphorous), and a canister shot.
The HEAT cartridge was a bit of a disappointment as the penetration was only 63,5mm , against 120mm for the Bazooka. The reason for this is the helmet shape of the cone with a pipe connected which was quite ineffective compared to the pointed straight cone for the Bazooka.
The cartridge depicted in this posting is the HEAT. The projectile body is made of drawn steel. A pre-engraved driving band is placed around the body of the projectile. The red copper shaped charge cone is crimped in place in a rim on the inside of the projectile body. A small steel pipe is placed below the cone, ending just above the booster charge pellet at the base of the explosive charge. The main charge consists of 177 grams pentolite 50/50, placed below the cone and around the pipe.
At the base of the pipe , the tetryl booster pellet is placed, fixated by the base screw. The base screw has a red copper washer between the flange of base screw and the base of the projectile. This to prevent hot powder gasses from reaching the booster pellet upon firing.
The fuze M90 consists of a cast and machined zamac (casting mixture of Zinc Alumium Magnesium and copper) housing, which is screwed over the top of the projectile body. In top, a nose impact detonator (red) is crimped in place. From the nose detonator, a hole runs downward, ending in the S&A mechanism.
The S&A mechanism consists of a brass rotor piece, housing the secondary detonator. In rest, this detonator is placed out of line with the hole in the pipe above, so with an accidental detonation of the nose detonator, the flame cannot reach the secondary detonator in the S&A mechanism. The secondary detonator is fixated in this position by two opposite pins from the centrifugal weight beside it which stick in two opposite holes in the rotor.
The centrifugal weight is a cup with two pins (protruding into the rotor), fixating it. The centrifugal weight has two slots at the opposite side through which a fixating pin is placed. This fixating pin ensures that the centrifugal weight can move outward radially, but cannot rotate like the secondary detonator. A spring inside the centrifugal weight forces the centrifugal weight inward, forcing the two pins into the holes of the rotor, fixating the rotor. Below the S&A mechanism, the main detonator is placed. Upon activation, it will spit a flame down the pipe, which detonates the booster charge in the base.
Upon firing, the centrifugal force will throw outward the centrifugal weight, retracting both pins from the rotor. The rotor will throw outward the heaviest parts , causing rotation and placing the rotor detonator in line with the nose detonator and the main detonator below. Upon impact , the ballistic cap over the nose is crushed, as well as the detonator in the nose, which explodes. The flame travels down the pipe, igniting the detonator in the rotor, which on its turn- ignites the main detonator. The flame of the main detonator travels down the pipe below the cone , igniting the tetryl pellet and the main charge.
The Shellcase M30A1 is a drawn steel shellcase that has been perforated to allow gas escaping . The shellcase is filled with granular powder, ignited by a central tube filled with black powder.
In the neck of the shellcase three notches at 120 degrees each are stamped outward. They are meant as a forward stop and help to centre the shellcase in the forward end of the chamber. Older shellcases have a full brass ring which acts as a forward stop. As the driving belt is pre-engraved the projectile has to be rotated slightly when nearly fully inserted into the chamber until it fits the driving belt into the lands and grooves.
In older shellcases a paper liner on the inner wall of the shellcase keeps the propellant in, as well as dry and clean. Later (post war) shellcases use a polyethene bag as liner.
Length of the cartridge: 47,7cm
Weight of the cartridge: 2,54 kg
Length of the projectile: 19,9cm
Weight of the projectile: 1,25 kg
Length of the shellcase : 303 mm
V0: 365 mtrs/sec
Max range: 4350 mtrs.
Effective firing range : appr. 450 mtrs.
Penetration: 63,5mm steel at all distances. @ 90 degrees
The M18 was used by the following countries:
United states, Taiwan, China (58mm Type 36), North Vietnam (58mm Type 36), Philipines, Netherlands, Tanzania.
It must be noted that the Chinese version is not 57, but intentionally made 58mm calibre. Therefore , Chinese ammo cannot be fired from US manufactured weapons, but the slightly too small US ammo can be fired from the Chinese Recoilless rifle.
An interesting movie, showing weapon and ammunition can be found here:
https://www.youtube.com/watch?v=qsM8EtBZ-EQ
Regards, DJH
