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Originally posted by Colin Macgregor Stevens
The Polsten is a simplified version of the Oerlikon (far fewer parts 199 vs 250 and simpler design and cheaper i.e. GBP 60-70 vs Oerlikon GBP 320) developed by the Poles (thus the Pol in Polsten). Design gotten out of Poland in 1939. Finished in UK. Some reports say "sten" in Polsten is same as "Sten Company" but I seriously question this as I am not aware of such an armaments company. The Sten machine carbine was named after Shepherd & Turpin & Enfield but had no STEN company.
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As far as we can find out Polsten is simply the use of some letters from "Polish Type 20mm" or some such and "En" for Enfield again. Most likely it was cobbled-up as a good idea around the table having a mind to the origin of "Sten"
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The Hispano-Suiza 20 mm was used mainly by the RAF, RCAF etc. as wing guns in Spitfires, Hurricane IIB, etc. USAAF also used some e.g. as nose guns on some fighters. The British and Canadian Armies used some as well. They had DIFFERENT ammunition from Oerlikon & Polsten. Canadian Skink tank was originally to have Hispanos but was changed to Polstens as I recall.
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Looking at the drawings and by deduction only, it would seem the Hispano was solely designed and intended for aircraft installations where the infinite supply of cooling air allows a quite light-weight construction, and weight itself is of course very important in this application. The Oerlikons and Polstens are quite chunky by comparison.
Referring to the training manuals and description of operation here, which I admit to only glancing at so far, the Polsten operates much like a huge Sten and so is very simple. One fact that did stick in the mind from these original books (that Ballard has of course) is that the cartridge is fired 0.3" before it is fully chambered.
It talks of oiled rounds too which is usually a distinct disadvantage but a 20mm piece firing at the rate of a Bren is going some.
With the winter months coming I see I shall have to relieve Ballard of the books and make a close study to get my head around the exact detail.
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The guns on the CMP 15 Cwt trucks in Normandy - I identified that this is so far the only known photo of these purpose built CMP 15 Cwt with 20mm AA gun on the Continent which dispelled the standing myth in UK that they were never taken to France for action. I also identified the unit as being from the British 7th Armoured Division (Desert Rats.) The info by the owner of the only restored CMP 20mm unit (in UK) in the article in CONVOY had the wrong date (it was over a month AFTER D-Day) and wrong formation & Armd Div - I think article said 22nd Div.) I had spotted the photo, identified it, then sold the book containing the photo to the owner of the vehicle in UK.
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And what a gem that picture is, using the glass they are copiously marked, 21st AG, the Stags Head of 7th Armoured and therefore the Desert Rat too, then in a rare case of superimposed sinage the 73 of the 15th Isle of Man light AA regt is put onto the colours of the RA.
Through the net we have found regimental contacts in the IOM and Ballard is in contact, unfortunately their detailed history is in a now out of print book but we are looking.
R.