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 rifle s , 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 .