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The Gast was actually a very big and heavy gun, with a couple of huge drum magazines. The MG81Z "twin" of WW2 was much faster-firing and much smaller and lighter, as well as being belt-fed.
The Gast was thoroughly studied long before WW2, but no-one bothered to pick it up. I think that the problem may have been that it was too specialised. It couldn't be synchronised, and that was a major disadvantage at the time because most air forces preferred to use the same guns for fixed and flexible mounting (the British started out with the Browning and VGO, but rapidly dropped the latter)
While 1600+ RPM from two barrels was very good for WW I it wasn't so good by even the 1930s. Browning could do 1100-1200rpm for a single barrel, Vickers was up to 900rpm for the old belt feed gun, German MG 15 was 1000-1100rpm. Russians managed up to 1800rpm from their MG.
The guns were NOT independent. they were interlinked, as one bolt moved back the other moved forward, the bolts were connected by a link that was pivoted in middle of the housing. If one gun jammed both barrels stop firing.
A WWII 30mm Gast probably wouldn't have that rate of fire and/or would be heavier. But such a weapon would be a very viable and powerful option as a belly-mounted gun pod for Hs 129 in place of the BK 7.5 cm, I would think.
The Russian gun fires about a 20-22% heavier projectile about 3% faster than the 30mm gun the Hs 129 already used. Although the German AP round was closer in weight and had a higher velocity than the Russian gun.
Which 30mm? The Hs 129 B-1 first received the Mk 101 (960 m/s@230-250 RPM) and then the Mk 103 (940 m/s@420 RPM w/ AP rounds) in the B-2. Both had significantly lower rates of fire than any Gast design would have even if we assume a greatly reduced capability compared to the more advanced GSh-30-2. You might be thinking of the Mk 108, which never saw service on the Hs 129. That had a much higher (650 RPM) rate of fire but nowhere near sufficient velocity to do much to a tank.
The original Gast weighed 60lbs. A 1919 Browning ground gun goes about 31lbs with a heavy barrel. The aircraft versions use a lighter barrel and by the early 30s can fire 1200rpm. even if you get the original Gast up to 2400rpm from 1600rpm what do you gain?
The Gast is essentially two single guns linked together. Quite true, but it also means that it weighs as almost as much as two similar single guns. There is nothing in it's design that will significantly change the muzzle velocity ( beyond about 1% or less) of a given round fired from it.
You also have the problem that if one barrel has a misfire both barrels stop firing until the gun can be "charged". If one barrel suffers a feed malfunction both barrels stop firing. Assuming the feed malfunction doesn't foul the bolt your "single shot" capability comes from using a hand charger, electric solenoid, pneumatic cylinder or hydraulic cylinder to move the bolt after every shot. Useless for air to air combat and near useless for ground attack.
The Russians may like them because they offer a good rate of fire from an easily manufactured gun. Given the Russian preference for almost "throw away" guns that give good performance for weight it may be understandable.
machine guns weren't ever lacking for ROF even by the 1930's but during and after BoB they were largely useless against bombers and not even very good against fighters.
The crews of the 424 bombers (excluding Ju 87s) lost between July and September 1940 as a result of enemy action,most being shot down by the RAF's fighters would probably disagree.
The crews of the 398 single engined and 214 twin engined Luftwaffe fighters lost to enemy action in the same period would also probably disagree.