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.
| Rate of fire | 450rds per min |
| Effective ceiling | 6630 ft |
| Muzzle velocity | 2725 ft/s |
| Rifling | Concentric RH twist, 1 in 720 |
| Magazine capacity | 30rd box or 60rd drum |
| Overall length of gun | 84" |
| Length of barrel | 57" |
| Weight of gun | 126lb (57kg) |
| Weight of 60rd mag empty | 31lb 8oz (14.3kg) |
| Weight of 60rd mag full | 64lb (29kg) |
| Weight of 20mm round | 8oz (225g) |
| Date of manufacture | 1944 |
| Manufacturer | John Inglis Company, Canada |
| Mounting weight | 3910lb (1777kg) |
| Platform weight | 1320lb (600kg) |
| Carriage weight | 1665lb (757kg) |
| Total weight of quad gun | 6895lb (3134kg) |