High-Low System
| Pansarskott m/68 "Miniman" | |
 Pskott m/68 from Swedish Army manual
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Miniman high-low launch system located behind 74mm HEAT projectile
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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. It enables a much larger projectile to be launched without the heavy equipment typically required for large caliber weapons. 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.
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.
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Wikipedia