Aha, my favourite subject
if you want to read about the comparisons between .50 and cannon, then
THIS article discusses the pros and cons. If you want more technical, direct comparisons between WW2 ammunition, guns and aircraft installations, then
THIS gives you all the gen I could put together.
On the specific subject of the MK 108, I can contribute the following: it appears that the MK 101, MK 103 and MK 108
could all fire the same projectiles, although they usually didn't.
The MK 101 was much older than the other two, and usually fired conventional AP and HE ammunition (with projectiles weighing 433-500g), although towards the end of its life it was also loaded with the tungsten-cored Hartkernmunition (355g) for anti-tank use.
The MK 103 was used for two different purposes: first, to replace the MK 101 in the anti-tank role (carried by the Hs 129) in which instance it usually fired the Hartkern rounds (although the ammunition was not interchangeable with the MK 101 - it had the same 30x184B dimensions, but used electric rather than percussion ignition), although sometimes conventional AP; second, to arm heavy fighters for the anti-bomber role, in which case it normally used M-Geschoss ammo (330g shells).
The MK 108 was designed for the anti-bomber role and was, in fact, designed for M-Geschoss projectiles from the start. Judging by the number of surviving examples, the great majority were HEI but some M-Geschoss shell bodies were loaded with incendiary material instead. For some strange reason, a few have been found loaded with Hartkern projectiles.
As far as ballistic performance is concerned, it is important to note that there were two different shapes of 30mm M-Geschoss shells (both types are found in 30x90RB MK 108 and 30x184B MK 103 cases). The original type, the Ausf. A, had the parallel-sided shell body and blunt nose fuze as already shown - this carried no tracer. The later Ausf. C was far more streamlined and carried a tracer. It lost a little in HE capacity but retained its original velocity far better: when fired from the MK 108, the Ausf. A had lost 47% of its muzzle velocity at 600m, but the Ausf. C had lost only 26% (for comparison, the 20mm M-Geschoss lost 60%, the 20mm Hispano 43% and the .50 Browning 29%). The Ausf. A is far more common among surviving rounds.
The pic below shows the 30x90RB loaded with the Ausf. C type (the 30x184B being loaded with the Ausf. A):
Tony Williams: Military gun and ammunition
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