It's tough to take more than a step or two into official material on WWII aircraft weapons without running into concerns about deflected bullets. I won't be able to comb through for the best, most relevant excerpts but I did a quick skim through a few documents and scooped up what I came across.
Comparison of British .303" A.P. and German 7.92 m.m. A.P. ammunition
(Orfordness, October 1940)
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(3) Attack of armour through duralumin structure.
The bullet on meeting aircraft duralumin skin and structure is deflected and retarded. This has a pronounced effect on the subsequent damaging capacity of the bullet. The magnitude of the deflection depends on the design of the bullet and on its degree of stability. The evidence tabulated below indicates that the German bullet is more adversely affected by impact on the aircraft structure than the British bullet.
In the testing referenced in this report, a Blenheim was attacked at 200 yards from dead astern using each type of ammunition. The rounds striking the rear fuselage within the projected area of the gunner's armour were examined.
Percentage deflected away from and out of the fuselage: British: 38%, German: 29%
Percentage reaching armour through the fuselage: British: 33%, German: 23%
For comparison's sake, here are the deflection percentages of British ammunition from different angles of attack:
10° off: 8%
10° above: 24%
10° below: 2%
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Notes on Types of .303 Ammunition for use in Aircraft
(Gunnery Sub Committee, Jan 1939)
(i) Mark VII Ball has reasonable penetrative capacity against light metal structures, but it is readily stopped by armour plate or heavy fittings. At flat angles of impact, e.g. astern attack, about 30% of the bullets are deflected off the skin of modern aircraft.
(ii) Mark I A.P. is slightly more susceptible to deflection at flat angles than Ball, but has superior piercing qualities when confronted by armour plate or heavy fittings at steep angles of impact. This ammunition causes rather heavier barrel wear than Ball ammunition.
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Recent trials have proved that only a small percentage of Ball or A.P. .303" ammunition fired at a modern bomber from astern penetrates sufficiently to do vital damage, and that A.P. is slightly less effective than Ball owing to its greater susceptibility to deflection.
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The estimated vulnerability of a F.W. 190 A. 3 from direct astern
(Orfordness, Dec 1942)
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The angle of the fuselage with the line of attack is 6° and this gives the 0.303" B. Mk VII and 0.5" B. Mk.II no chance of causing a fire. These ammunitions will be for the most part deflected or broken up on the 19 s.w.g. fuselage skin; angles off astern would make these ammunitions more effective.
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The vulnerability of the German Flying Bomb to fighter attack
(Farnborough, Jun 1945)
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As a result of ground firing trials a variety of 20 m.m ammunitions, of both Service and experimental types, against flying bomb components the following conclusions were reached by C.E.A.D.
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(ii) Plain A.P. 20 m.m. ammunitions may be rather less effective than the less rigid 20 m.m. S.A.P./I. owing to the greater ease with which the former type appears to be deflected.
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Firing Trials of Manganese steel deflector plates.
(Orfordness, January 1942)
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The deflecting properties of 18 and 20 s.w.g. manganese steel, when attacked with 0.303" W. Mk.I A.P., are comparable with 14 and 18 s.w.g. respectively of dural. (For what it's worth, as far as I can tell with limited research, 18-22 s.w.g dural is a reasonable average for most WWII aircraft skins)
Critical angle of deflection
18 s.w.g.: 8½° – 9°
20 s.w.g.: 5°