The nature of the structure did make quite a difference to the effect of HE shells. This is from 'Flying Guns - World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45':
"The 30 mm HEI M-Geschoss, fired from an MK 108, was also tested. Unsurprisingly, ten rounds fired at a Spitfire fuselage resulted in a score of three immediately lethal, seven probably lethal. Eleven rounds fired at a Blenheim achieved the same results, plus one doubtful. It was noted that the ammunition did not have much effect on heavy bomber fuselages (presumably because of the large volume for the explosion to dissipate into), but inflicted serious aerodynamic damage to the wings by blowing off the surfaces, and that the incendiary content was very effective in starting fires. German tests reflected these results, and also revealed significant differences in the effectiveness of the mine shells depending on the construction of the aircraft. Stressed-skin alloy monococque structures were most vulnerable to being blown apart. Steel structures clad with thin aluminium were less affected as the cladding quickly split, releasing the pressure before it had much time to damage the structure, and fabric-covered structures were damaged least of all."
Tony Williams: Military gun and ammunition
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