OK - then if one the armor was tested, that's one thing, but if you have live rounds, "deflecting" (not penetrating) the skin, that's a different situation as these deflections may cause internal structural damage to load carrying components that have the possibility of failure.
Post #17 is by another person
Well that's my point - the Rand study was a bit cleaner as the physical evidence was examined (if we're talking about damage from flack). Making a determination after an aircraft fell from the sky is more difficult as the evidence of what actually brought down the aircraft may be (for better choice of words) "contaminated" by other factors indicated (ex. fire)
As the person in question, I will state that the tables posted by Grayman are based on data taken from the Rand Aircraft Vulnerability Study
Here is the preamble in the Rand report to the table I reproduced from that report.
"The most detailed information now available relative to the causes of aircraft loss and damage is a summary report prepared by the Evaluation Section, Air Branch, office of Naval Intelligence, which is based on a study of those Naval and Marine aircraft lost or damaged to Japanese action between September 1944 and August 1945. During this year 2,335 Naval and Marine aircraft were destroyed on action sorties in the Pacific and roughly twice that number were damaged, with 66.3 per cent of the destruction and 80.8 per cent of the damage attributable to enemy action. For 93 per cent of the aircraft lost and for all the aircraft damaged in this period at least partial data were available from AC-1 reports, Ship's War Diaries and Action reports."
"Carrier based aircraft included the F6F, FM, F4U, SB2C, and TBM; land-based the F6F, FM, F4U, SB2C, SBD, TBM, PBY, PBM, PBJ, and the PV.
Nearly half (45%) of the aircraft lost to enemy action were known to have been lost because of hits on the flight controls, or oil, fuel, hydraulic, electrical or water injection systems. The component damage reported to have resulted in the loss of aircraft is shown in Table 9 on the following page.
Of the lost aircraft that went down immediately, 54 per cent were due to hits on the pilot and/or flight controls and 25 per cent to hits of the fuel system. No data is available as to the fires occurring on these aircraft. Hits on the powerplant or oil system were responsible for 68 per cent of those aircraft forced to make a controlled water landing or bail out near the target. Damaged structures and hydraulic systems accounted for 73 per cent of the aircraft which were able to make a landing at base but which were damaged to the extent that they were stricken from Navy records."
As a point of interest on the 50 cal vs 20 mm debate here is what the report has to say:
"Serious damage is defined as that which had significant influence on the effective operation of the aircraft as a military weapon, slight damage that which had no significant influence.
The above table shows that of damage on returned aircraft, approximately one-third of damage due to anti aircraft fire, one-fourth of damage due to .30 and .50 caliber fire and two- thirds of damage due to 20 mm fire was serious."
Here is my recreation of the table
I did find an error in the original math which I have corrected.
If you do the math to separate the effects of the .30 and .50 calibers you will see that one-fifth of damage due to .30 caliber fire and one-third of damage due to .50 caliber fire was serious. Regardless, it does speak to the greater hitting power of the 20 mm.