Finn,
A delayed reply to your question, due entirely to my inability to read the thread properly - my sincere apologies.
The Pk of a cannon round is a combination of the characteristics of the round and the target, and how the round arrives (how fast, and from what direction). (I'm not talking about solid bullets here). The earliest shells designed to shoot down aircraft relied on straight blast, as they were dealing with fairly fragile structures. The main challenge was to get the round to explode as it passed through light structures - you had to get a hit on a hard object (engine, metal systems) for them to go off.
As a result, shell designers developed very sensitive, fast acting fuses with good 'grazing initiation' performance, and they also designed the cases to produce a large number of small fragments to maximise the chance of causing damage. Finally, they loaded the shells with materials that produced sparks and flame, with the aim of setting off fires or igniting fuel.
The Germans applied the most science to this rather arcane art, and after experiments on crashed Allied bombers, changed their cannon designs to take a new larger 30mm round designed specifically to knock down B-17s and B-24s by exploding near the wing roots. That very specifically designed round was taken on as the Aden 30. The Mauser 27mm shell, developed by Diehl, still uses a similar design philosophy of a high level of blast pressure plus a large number (over 400) of fragments to blow structure apart.
Pk of cannon shells against fighter aircraft, or more modern aircraft, was an area of research that was relatively neglected for some years. However, in the 70s and the 80s, the Norwegians did a series of trials on old F-86s, which led to a new concept for shell design. They went for a shell that produced a smaller number (about 35) of bigger fragments, and these fragments are blasted forward down a 30 degree cone, rather than a nearly spherical fragment cloud.
The aim of all this was to give a bigger chance of damaging critical components such as the engine and other 'hard' systems, with the shells arriving from behind the aircraft. They also realised that a very fast acting fuse might not be as effective as one that allowed the bullet to get deeper into the aircraft before going off, so they developed a clever system of variable fuse timing. This process resulted in the Raufoss 'MultiPurpose' (MP) family of rounds. They produced an MP 27mm round, but I don't think it was taken up.
These are really specialised areas of study, and there are major arguments over the 'science' behind it all. As I hope this shows, what sounds like a simple issue can be slightly more complicated.
Hope it helps
Best Regards
Engines