I appreciate what your saying, but again, not so fast. there are various things that the .303 has to pass through just to get to the engine. not to mention the angle that it hits the engine at. realisticly, how many .303 bullets fired in combat conditions would penetrate the block? 1 in 10? 1 in 100? 1 in 1000? 1 in 10000?
I think that's more to the point regarding 7mm rounds, however i take the word "capable" to mean that it can, but not all the time.
I show a ball round of 7.7mm going through 8mm of armor at 200 yards, and M2 50 cal going through 20mm at the same range.
At 600 yards the M2 still penitrates up to 16mm of armor plating and that's where your benefit is in having a heavier round.
Need i address 20mm AP at 600 yards?? ok... that goes through 23mm at that range.
However the HE 20mm (Hispano Mk I) doesn't get beyond 8mm at 200 yards.
You can make guesses on oil pan shots or cracking engine blocks, and the answer would be that the 7.7mm is capable.
However any off angle shot gave no results, where the 50 cal still penitrated beyond 12mm at the same range.
For that reason pilots probably saw more consistency with their shots over a variety of ranges as well as a variety of targets, something where 7.7mm probably would have a much narrower use in air to air or air to ground applications.
You can find that data on this forum under the 50 cal vs 20mm thread.