This is from 'Flying Guns: WW2':
"Another thorny issue was harmonisation. At the beginning of the war, the official Fighter Command practice dictated a convergence point at 400 yards (366 m). This “Dowding spread” was intended to increase the probability of hits: At typical fighting distances, the guns covered an area roughly corresponding to the distance between the engines of a twin-engined bomber. It was reasoned that inexperienced pilots and those with less shooting ability profited from having dispersion at normal combat distances. A similar pattern was recommended by the Air Ministry. This was a shotgun approach, based on the assumption that a few hits would be sufficient. But trials in December 1939 and early combat experience indicated that the strong metal construction of modern combat aircraft, with stressed skins and self-sealing fuel tanks, was able to absorb a surprising number of hits. (Later several German bombers would return home with as many as 200 rifle-calibre hits.) This necessitated a switch to convergence at a shorter distance, to concentrate the hits in a smaller area.
A number of squadrons harmonised their guns to converge at 350 yards (320 m) or 250 yards (230 m), and the latter option was preferred by expert pilots. In March 1940 Dowding finally decided to change the harmonisation distance to 250 yards, although the evidence from combat experience and wreck examinations was still too incomplete to be decisive. The argument was not settled until June, when combat reports indicated a significantly higher (53% vs. 39%) success rate with the shorter harmonisation distance.
This was not a definitive end to the debate. When more powerful armament was installed, advantages were again seen in some dispersion of fire. The strongest argument in favour was that very flew pilots were able to judge correctly the amount of lead required for deflection shooting. In May 1942 the RAF would again opt for a greater dispersion of fire to generate more hits."
There are also some official diagrams showing the effect on a target (He 11 at 6 o/clock) at different ranges included in the book.
It is worth emphasising the point that Henning makes. Just because the guns were initially harmonised at 400 yards didn't mean that was the expected combat distance; it was assumed to be the maximum distance, with reasonable concentration of fire between the expected 200-400 yards firing range. In fact, they had to get much closer than they originally thought they would.
It is also worth mentioning that the protection provided to German planes seems to have been steadily improved during 1940.
As far the effectiveness of .303 ammo is concerned, this is also from 'Flying Guns':
"These results are supported by tests carried out by the British in January 1941 to compare British and German rifle-calibre steel-cored AP ammunition. The performance of the .303" (11.28 g at 735 m/s) and the 7.92 mm (of unspecified type, but measured at 11.53 g at 788 m/s) was first tested against "homogeneous hard armour". The thickness necessary to achieve immunity from this ammunition at 183 m was 12.0 mm for the .303", 12.5 mm for the 7.92 mm, when striking "at normal" to the armour (i.e. at 90º). The British ammunition was significantly worse when the striking angle changed to 70º; only 6.6 mm was needed for immunity in comparison with 8.9 mm to protect against the German round.
The test then changed to shooting at the rear of the long-suffering Bristol Blenheim at the same distance, involving penetrating the rear fuselage before reaching the 4 mm armour plate protecting the rear gunner, which was angled at 60º to the line of fire. The results in this case were reversed; 33% of the .303" rounds reached the armour and 6% penetrated it. In contrast, only 23% of the 7.92 mm bullets reached the armour, and just 1% penetrated. The British speculated that the degree of stability of the bullets (determined by the bullet design and the gun's rifling) might have accounted for these differences.
The Germans helpfully tested other nations' ammunition as well, and this produced some surprising results. They rated the British .303" AP as capable of penetrating 9 mm / 100 m / 90º, but only 2 mm after first penetrating the angled dural skin (which contrasts sharply with the results the British observed). Comparable figures for the Soviet 7.62 mm API were 10.5 mm and 4 mm, but the API/T fell to a maximum of just 6.5 mm; the steel AP core was lighter as the tracer used up some of the space (a common disadvantage of small-calibre AP/T bullets)."
Clearly, even the alloy structure of a small plane like the Blenheim was capable of absorbing or deflecting the rifle-calibre bullets before they even reached the armour.
Tony Williams: Military gun and ammunition
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