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For example, the 7.9mm MG 17 mounted in the cowling of single-engined fighters was synchronised by means of an electrical firing system. It could therefore be described as "electric firing" but this is misleading. What happens in this case is that the synchro system sends an electrical impulse to a solenoid on the gun, which results in the firing pin being released to run forward and strike the percussion primer. The system is therefore electrically synchronised but percussion primed. In the ground role, a mechanical trigger replaces the solenoid and the gun can fire without needing a supply of electricity.
The MG 131 (and the electrically-primed versions of the MG 151 and MK 103, plus the MK 108) use a different method in which the synchro system sends an electrical pulse to the gun which instantly heats up the primer and fires the gun. So, no firing pin, and a different type of primer. Also, this system requires an electricity supply, which in ground use could most simply be provided by installing a battery on the gun mounting.
I think that your mention of "electro-pneumatic" applies to something entirely different. This is a method of charging the gun so that it is ready to fire. You will know that with any machine gun, before it will fire, a magazine or belt must be inserted and then the charging handle is pulled to the rear and released; this pulls the bolt back and aligns the next round with the gun chamber. If the ammunition proves unreliable, it may be necessary to do this whenever the gun stops firing in order to extract and eject a faulty round. In WW1 aircraft the cowling guns were usually mounted so that the pilot could easily reach and operate the charging handles. However, this obviously could not happen with wing-mounted guns or some cowling installations, so a powered system was installed so that the pilot only had to press a button to re-charge the gun. This system could be electrical, or pneumatic, or some combination of both.
Interestingly, one of the modifications made to the shorter and lighter Mark 5 version of the RAF's 20mm Hispano was to omit the powered charging system; the cannon were charged on the ground by a mechanic. Evidently the ammunition was considered to be reliable enough not to need the system.
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Found this in NARA T78 R623 p053.
Document dated 10.12.44.
800 MG 131 in Plexiglaskuppel for the Panzertruppe.
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