Polsten 20mm Quad

Polsten 20mm Quad

History and Design

The Polsten machine gun is a 20mm weapon which has been designed primarily to fulfill an anti-aircraft role against low flying aircraft and pilotless planes. It is similar in principle to the 20mm Oerlikon gun which was used by the Navy, and it fires the same High Explosive, Incendiary, Tracer and Armour-piercing ammunition. This makes it an extremely effective weapon when employed in the A.A. role. In a secondary role it may be used with good effect against ground targets, e.g. soft skinned transport and lightly armoured vehicles.
There are several mountings on which the Polsten was used; No2 Universal mounting, Mk1 Haszaird-Baird mounting, 20mm triple mounting and the Quadruple mounting etc, but it could be fitted, with slight modification to the firing gear, to other mountings on which the Oerlikon is normally carried.
A spare barrel is provided with each gun. The barrel can be changed quickly and easily, and this was done frequently in order to reduce overheating and consequent barrel wear.
The Polsten magazine was a 30 round box type and was not really suitable for use on the quadruple mount so the 60 round Oerlikon Mk2 drum magazine was used, which weighed 64lb when filled with ammuntion ! Each 20mm round weighed about half a pound.
The Polsten machine gun had an unusual deleopment history, for it´s design originated in Poland and was transferred to the UK when the Germans invaded Poland in 1939. The Polish team had been revising the basic Oerlikon gun with a view to simplifying the manufacture and reducing cost. The drawings were completed by a team of Polish, Czech and British designers. The design was then handed over to the British Sten Company (name not proved) who produced the gun along with the John Inglis Company in Canada. It was cheaper than the Oerlikon for it cost £60 -£70 compared with £320 for the Swiss original. This was due to the reduced number of components, 119 as opposed to 250, and the smaller number of machining operations to turn them out. The first 240 guns were produced at the end of 1943, and a further 13,548 during 1944.

Development and Application

The design of the quadruple mounted Polsten was developed along side the "Skink", a modified M4A1 Grizzly medium tank which had a specially designed turret, using 4 Polsten Mk1 machine guns instead of the usual armament. The purpose of the Skink was to provide protection for armoured units and assault troops against low flying aircraft. The original program called for production of 135 complete skinks for the use of the Canadian Army and an additional 130 Skink turrets to meet British requirements.
Allied air supremacy in North West Europe eliminated the demand for this type of vehicle and on 23rd August 1944 the Dept of Munitions and Supply was advised by the National Defence that the program should be cancelled and only three tanks would be required. Of the three vehicles produced in Canada, one actually saw service in NWE during Feb and March 1945. Although it wasn´t actually used in the A.A. role it was very effective in the ground role.
However, another threat loomed in 1944, that was the use of V1 flying bombs or "Doodle Bugs " against Southern England by the Germans. Conventional A.A. guns such as the 3.7´s and 40 mm Bofors were found to be inadequate to deal with the problem so a new A.A. gun was produced based on the Skink turret design. The gun could be trailed into position anywhere quickly and then set up. A team of loaders would keep the ammunition coming and the gunner would sit inside the armoured turret and direct the fire using a joystick similar to an aircraft type control. The action was very fast, being hydraulically controlled, hydraulic power produced by a small Wisconsin single cylinder petrol engine in the rear of the turret. The gunner had elevation control as well as traversing and could fire the guns as pairs or all at once ! A contract was placed for 1000 quad Polstens and they were delivered. The quadruple mount also saw service in NWE when it was used to defend Antwerp from the V1s. A lorried version was also built using the turret and mounting it on the back of a CMP flat bed 3 tonner with a type 43 cab.

Physical data
Rate of fire450rds per min
Effective ceiling6630 ft
Muzzle velocity2725 ft/s
RiflingConcentric RH twist, 1 in 720
Magazine capacity30rd box or 60rd drum
Overall length of gun84"
Length of barrel57"
Weight of gun126lb (57kg)
Weight of 60rd mag empty31lb 8oz (14.3kg)
Weight of 60rd mag full64lb (29kg)
Weight of 20mm round8oz (225g)
Date of manufacture1944
ManufacturerJohn Inglis Company, Canada
Mounting weight3910lb (1777kg)
Platform weight1320lb (600kg)
Carriage weight1665lb (757kg)
Total weight of quad gun6895lb (3134kg)