High–low system

The High-Low system also referred to as the "High-Low Pressure system", the "High-Low Propulsion System", and the "High-Low projection system", is a design of cannon and antitank launcher using a smaller high-pressure chamber for storing the propellant. When the propellant is ignited, the higher pressure gases are bled out through vents (or ports) at reduced pressure to a much larger low pressure chamber to push the projectile forward. With the High-Low System a weapon can be designed with reduced or negligible recoil. The High-Low System also allows the weight of the weapon and its ammunition to be significantly reduced. Manufacturing cost and production time are drastically lower than standard cannon or other small-arm weapon systems firing a projectile of the same size and weight. It has a far more efficient use of the propellant, unlike earlier recoilless weapons, where most of the propellant is expended to the rear of the weapon to counter the recoil of the projectile being fired.

Origin
In the final years of World War II, Nazi Germany researched and developed low-cost antitank weapons. Large antitank cannon firing high velocity projectiles were the best option, but expensive to produce as well as requiring a well trained crew. They also lacked mobility on the battlefield once emplaced. Antitank rocket launchers and recoilless rifles, while much lighter and simpler to manufacture, gave the gunner's position away and were not as accurate as antitank cannon. Recoilless rifles used a huge amount of propellant to fire the projectile, with estimates ranging from only one-fifth to one-ninth of the propellant gases being used to push the projectile forward. The German military asked for an antitank weapon with performance in-between that of the standard high velocity cannon and the cheaper rocket and recoilless infantry antitank weapons. They also stipulated that any solution had to be more efficient in the use of propellant as German war industry had reached maximum cannon propellent production capacity.

In 1944, the German firm Rheinmetall-Borsig came up with a completely new concept for propelling a projectile from cannon, which, while not recoilless, greatly reduced recoil and drastically reduced the manufacturing cost. This concept was called the Hoch-und-Niederdruck System which roughly translates to "High-Low Pressure System". With this system, only the very back of the cannon's breech had to be reinforced against the high firing pressures.

Rheinmetall designed an antitank cannon using their "High-Low Pressure System" that fired a standard general purpose HE 8.1-cm mortar bomb which had been modified to function as an antitank round with a shaped charge. Normally, a mortar bomb cannot be fired from a cannon, because their thin walls can not endure the high stress forces upon firing from a cannon. The 8.1-cm round was mounted on a rod which was fixed to a round steel plate with eight holes in it. A shear pin held the round to the rod. The round and the plate were fitted at the mouth of a cut-down cannon shell casing which contained two propellant bags. On firing, the pressure would build up in the shell casing, which along with the reinforced breech, acted as the "High Pressure Chamber" and bled out the steel plate holes at half the pressure to the thinner walled cannon barrel which acted as the "Low Pressure Chamber". Unlike standard cannon, in which the propellant "kicks" the projectile out the barrel with an almost instant acceleration to maximum muzzle velocity, the Rheinmetall concept "shoved" the projectile out the barrel at a constantly increasing muzzle velocity. There was recoil, but nowhere near the recoil of the 5-cm and 7.5-cm antitank cannons being used at that time by the German forces, which required a heavy constructed carriage, and both a heavy and complex hydraulic recoil mechanism as well as a muzzle brake to contain the massive recoil on firing. The Rheinmetall solution required only a lightweight recoil unit and muzzle brake. The only major drawback was its maximum range of 750 meters (in direct fire against tanks), but this was offset by an armor penetration of 140 mm and no telltale back-blast. The Germans ordered the Rheinmetall gun into production, designating it as the 8-cm Panzer Abwehr Werfer 600 (PAW 600). Only about 250 were produced before the war's end. None were reported to have seen combat.

Further development
The Allies captured and examined the PWK, but initially showed little interest in the new system developed by the Germans. The first example of a type of High-Low System developed after World War II was the British Limbo antisubmarine weapon, which launched depth charge-like projectiles. The Limbo was a development of the World War II Squid, which, while effective, was limited by a set range of 275 meters. The Limbo, by opening and closing vents that varied the pressure of the gases on firing, allowed for a range that could be varied between 336 meters to almost 1000 meters.

M79 40-mm grenade launcher
The most well known use of the High-Low System was by the U.S. Army with the introduction of the M79 grenade launcher shortly before the Vietnam War. The M79 fired a 40-mm shell which contained a standard fragmentation grenade with a modified fuze. The cartridge casing contained a heavy cup-shaped "High Pressure Chamber" in the bottom. On firing, the propellant builds up pressure until it breaks through the copper cover, venting out to the "Low Pressure Chamber". The U.S. Army referred to their high-low system as the High-Low Propulsion System. Along with a heavy rubber pad on the M79 butt stock, the High-Low system kept recoil forces manageable for the infantryman using the weapon.

The M79 was later replaced by the M203 which mounts under the barrel of an assault rifle. Later, the U.S. Army developed a higher velocity 40-mm round using their High-Low Propulsion system for use by heavier machine gun-type grenade launchers used on vehicles and helicopters. Today, besides the U.S. military, the 40-mm grenade family is extremely popular and in use by armies worldwide and variants of it are in production by countries other than the U.S., with one reputable reference publication in 1994 needing almost a dozen pages to list all the variants and nations producing 40-mm grenade ammunition based on the U.S. Army's development of the 1960s.

Russian developments
Shortly after the Vietnam War ended, the Soviet Union introduced a 40-mm grenade launcher that used the High-Low Principle, but with a twist on the original design. The GP-25 40-mm grenade launcher fits under the assault rifle and fires a caseless projectile that is muzzle-loaded. Instead of having a case, the high-pressure chamber is located on the rear of the projectile with ten vent holes, in which the launcher barrel acts as the low-pressure chamber. The ignition of the propellant gases also causes the drive band to engage the launcher grooves, similar to the American Civil War Parrott muzzle loading rifled cannon.

While little documentation exists, in the 1950s the Soviet Army developed 73-mm cannon for wheeled armored reconnaissance vehicles that fired a munition very similar in operation to the original World War II German concept. However, it was never introduced into service, and instead the Russians developed a low velocity 73mm cannon that fired a rocket projectile which was ejected by a small charge in the normal fashion.

Swedish use
Besides the previously mentioned family of popular 40-mm grenades, the only other major use of a High-Low System was by the Swedish firm FFV in their development of the 1960s-era Miniman one man infantry antitank weapon. The Miniman was simpler and cheaper than anything imagined by designers in World War II. Inside what looked like a rocket launcher tube, is a HEAT projectile attached by a break away bolt to an alloy aluminum tube with ports drilled in it and which acts like a kind of high-pressure chamber. The launch tube in which it is mounted acts as the low-pressure chamber. When the propellant is ignited in the aluminum tube, gases escape through the ports and are allowed to build up in the launch tube to the point of almost causing a recoil. The break away bolt then snaps, allowing the projectile to move forward. Unlike other High-Low Systems, gases are allowed to escape to the rear of the launch tube, achieving a totally recoilless effect.