I am not sure, if you even need 25mmm in this particular case. I would say that with the right aiming even a machine gun will do it.

But when you do not look at the penetrating power alone, but the blowing effect, than a bigger round can hold more explosives (of course, I don't know, if the A-10 Gatling cannon can also take explosive rounds instead of the armor breaking rounds).

But of course the parallel poster is right: there are several things to penetrate -- particularly when partisans are heavily covered.