3.7cm Bordkanone

Was it derived from 3.7cm Flak 18 or newer 3.7cm Flak 43?

From pre-war weapons. It was operating by short recoil, firing at 160 rpm cyclic.
The Flak 43 was gas operated, firing at 250 pm cyclic. According to Tony Williams.

BTW, Dave, I think I have the answer why the Czechs installed double magazines instead of belts, in their take at 30mm AAA. The cartridges were to be lubricated ( by a mixture of oil and grease, we've called that 'emulsion') in order for cannon to work properly. Much easier tidier to manipulate, than messing with greasy heavy belts.

Why didn't they use the newer 3.7cm Flak43?

IMO the main benefit (higher RoF) meant nothing for the purpose - the gun was to be employed more akin big sniper, than a big MG. The feeble ammo supply would last longer, too.
The ground based Flak needed all the RoF it could muster, on the other hand.

davebender said:

Why didn't they use the newer 3.7cm Flak43?

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I assume the FlaK 18 was available and known as reliable whereas the FlaK43 was still in development/prototype status.

I'm under the impression 3.7cm Flak43 evolved from belt feed 3cm Mk103 cannon. If so then a belt feed variant of 3.7cm Flak43 should also have been possible. That would solve the limited ammo supply problem.

davebender said:

I'm under the impression 3.7cm Flak43 evolved from belt feed 3cm Mk103 cannon. If so then a belt feed variant of 3.7cm Flak43 should also have been possible. That would solve the limited ammo supply problem.

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Except that the 3.7cm Flak43 didn't have a belt feed which means all the detail work would have to be done. Simply scaling up all parts is just a start. It also doesn't solve the weight problem. Each round of 37mm ammo weighs about 1.5kg. Each round of 30mm ammo (HE) was 815 grams.

And what do you get for all this work? 160rpm is 2.66 rounds per second. 250rpm for the Flak 43 is 4.16 rounds per second. A 3 second burst from the Flak 18 is 8 rounds (OK 7.98 ) rounds. From a Flak 43 it is 12.5 rounds. That is if the trigger is held down and not if the guns are used as semi automatics. chances of still being on target after 3 seconds are slim and none. The faster firing guns will transmit more recoil to the plane and if not ON the flight/thrust axis will cause the nose to dip more.

More rounds fired that miss is not an increase in firepower.