Hello Kurfürst
I assume that the Hisso's shell, being heavier and faster, was better against armour and so more dangerous to things behind the armour. Also heavier shells produces higher weight of fragments or it has more HE inside, usually both. And heavier and faster shell penetrates deeper into target before exploding if fusing is same.
That's the part I don't get - its not a AP or solid shell. Its a shell that blows up upon/very shortly after impact. As a result
1, Its capability is marginal against armor because of its construction, and because by the time it would reach armor the fuse blows it into tiny bits of fragments.
2, Heavier and/or faster does not particularly effect how 'deep' it penetrates into the fuselage (as this a matter of fuse timing, achieving a desired distance) and, with the fuselage being a hollow, very thin layer of aluminium skin and not some body or piece of dense, solid structure, its isn't particularly hard to punch true - I guess even a .22 LR round would be more than capable of that,
not to speak of the capabilities a 20mm round.
And course, the amount of damage done is proportional to the amount of energy required to yield that damage to the structure. In the case of the same damage, the energy is the same and very likely very much smaller than the total energy the shell possesses, given light aircraft structure.
For example, if lets say, a spar flange requires 2000 Joules to have punched a 20mm diameter hole in it, in practical matter it doesn't really matter if you have 20 000 or 25 000 Joule KE round for the task, since the only difference will be that, to simplify matters, that the less powerful round will continue its path once it exited the aircraft with 18 000 Joule, while the other still has 23 000 Joule that can used for nothing else but a short and uneventful journey until it falls back to ground (or self destructs).
A heavier HE shell of course can produce more/larger fragments, but the potential of these is fairly low, usually stopped within a few layers of 'stuff' getting in their way. In any case, the KE of the fragments is dependent on the amount and energy of explosives within the shell (from where it gets its source KE), and again has no relation to muzzle velocity or shell weight.
Overall, a HE shells does damage in the following way:
1, Some damage as it enters structure, breaking things on its way, and the damage done equal
the energy required to destroy the particular thing, the remainder is just
surplus KE and plays no 'useful' purpose sine the projectile will blow up soon.
In this regard there won't be too much difference between a high powered and lower power round, because they both possess far greater energy than it is required to penetrate, but this KE will be soon equal to zero as
2, the projectile blows up, and it looses its KE energy. Then it will:
2a, Send small fragments in every direction with very little individual damage capability, but covering a wide area, having enough KE to damage the first light structure/component they hit (lines, wires, skin, crew). This KE is derivied from the amount of explosives in the shell - some part of which is consumed to break up the shell body into fragments. Fragments usually stop in the first thing they hit.
2b, Pressure and hot gases that again comes from the amount of explosives in the shell and will be capable of large amount of damage, quickly diminishing with distance but effecting everything nearby. Now, IIRC the Hispano HEI shell actually contained only half explosives, half incendinaries.
Actually that's why I asked - 'why'. The physics behind it simply don't explain (at least for me) how higher muzzle velocity, or higher projectile weight, can translate into higher damage done, especially as after the projectile blows up initial KE becomes irrelevant and most of the damage is done or directly related the chemical energy from the explosive charge.
Higher muzzle velocity, or higher projectile weight would factor in if we would speak of non-fused, solid rounds (like AP), but even with these there is very little
practical gain I can see - with penetration figures being in the 20+ mm if armor penetrated region, and most aircraft had 4-6-8-10mm on them at a few vital places, the rest being light structure, easily punctured by any round.
That doesn't necessary means that Hisso was better gun, only that the philosophies behind the two guns were slightly different, 151 counted more on blast and numerous small fragments and Hisso more on kinetic energy and fewer but heavier fragments.
Its not different design philosophy - the RAF simply had no other gun than the licensed Hispano, nor did it have M-Geschoss type rounds for it during the war. The French who designed it originally for a single gun installation, which meant that high ballistic performance gun - along with the bulk and recoil of it - in an engine installation, for which it was ideal. And if it was, the British decided after the war for adopting Mine shells into Mauser revolver cannons...
It is noteworthy that the Germans also considered originally a 20mm 'Elephant gun', the 20mm MG C 30 for the Bf 109 but instead opted for lightweight and compact Oerlikons in the wings. All of their later designs were compact with moderate ballistic performance, and Russian designs show similar considerations; it is noteworthy that these weapons were all purpose-designed for aircraft use, rather than adapted pieces originally meant for ground installations, so the designers must have understood that typical fighter combat ranges higher ballistic performance and bulk was less desirable than compactness and high rate of fire.