The Polsten design originated in Poland and was transferred to the UK when the Germans overran Poland in 1939. 

A Polish team had been revising the basic Oerlikon gun with a view to simplifying manufacture and reducing cost. The design was completed by The Royal Ordnance Factory at Enfield. The name Polsten was an acronym of 'Pol' for Poland and 'Sten', 'S' and 'T' for Shepard and Turpin, the 2 British designers and 'En' for Enfield.

It was cheaper then the Oerlikon costing between £60 to £70 as opposed to £320 for the Swiss original. This was due to the reduced number of components, 119 as opposed to 250. The smaller number of machining operations required also helped reduce the cost of production too. 

The first Polsten guns were delivered in March 1944 and from that time were used wherever possible in place of Oerlikons, in tank designs and installations. Although primarily designed for limited air defence their rapid rate of fire proved them very effective ground weapon. The Polsten had a number of original design features among which was the unusual "double stack" box magazine, holding 30 rounds. Guns used by the Airborne Forces used a 60 round drum magazine.

There were 2 guns per Section each towed behind a Jeep. Both gun and Jeep could be transported together in either a Horsa glider or Dakota aircraft.    

The Polsten Section of the 1st Airborne Reconnaissance Squadron was a unique unit within a unit. It was one of the few units to operate the Polsten in an airborne operation during the campaign in North West Europe.  

"Jack" Watson related how the Polsten  Section had their first engagement on the 17 September 1944 during the battle for Arnhem. 

"We didn't see the damage we caused Gerry or how many were killed, but our gun rate was eight 20mm shells slammed into a target per gun every second.......The Polsten was specially wheel mounted and towed - barrel backwards - by a Jeep. This fact was not a happy situation when caught in tight narrow spaces. The gun being towed backwards had to be unhitched from the Jeep and the Jeep driver get himself and and his vehicle out of the way pretty damn quick and if you cannot squeeze past either side of the gun by adriot and dextrous stratagems there is only one other way out - towards the enemy, who if wide awake is probably throwing stones or something at you by this time."

During this battle the Polsten had:

Muzzle velocity 2,700ft/sec   831m/s .

Ammunition was 20mm/0.787in Magazine holding 60 rounds, which were loaded in sequence with tracer, armour piercing, explosive-head shells.

Rate of Fire was 450 rounds per minute

Ceiling Height (similar to Oerlikon) 6630 ft/sec    831 m/s

Overall length  85.75in  2178mm

Barrel Length 1448mm   57in

Weight of gun 54.9kg   121lb

Projectile weight (HE tracer) 0.119kg   0.2625lb

Weight of loaded 30 round magazine 13.6kg   30lb