When looking at small explosive bullets when does 1-3 grams of HE beat several thousand joules of kinetic energy of impact force?
What is cost of the small explosive bullet vs the cost of a non-explosive bullet in materials, machine time, labor? Times several million or tens of million.
Hmm, not sure where I was advocating HE shells for such small calibers. I'm not sure those make sense, particularly if you want a fuzed design. There were some designs with fuzeless HE, I guess with more sensitive HE that would explode upon impact. But for HMG class guns, probably something like API makes the most sense.
And we are back to the conflicting basic requirements. With faster aircraft as targets (and faster shooting platforms) you have less time to aim and less time that the target is within the firing area/impact zone. Do you worry about getting hits or the effect of the hits first?
Or with 2000hp engines (and jets are way above 2000hp) do you just stick in number of large, high velocity, fast firing 20mm guns (four 20mm guns at 750rpm each) and go back to research? Nations were still arguing during the 1950s.
Of course, everything is a compromise and a tradeoff. The ADEN/DEFA guns developed relatively soon after WWII had a MV of about 800 m/s, though generally it seems more modern aircraft guns have tended towards higher MV, something like 1000-1100 m/s seems to be the norm for current day designs. Of course modern jets also have the capability to lug around much heavier equipment compared to WWII planes. Though I wonder about the barrel wear, but maybe with modern guns used so much less maybe barrel wear isn't such a big issue anymore.