The only thing I can think of is that Lancs (and Halifaxes) had more weight in their defensive armament with the total weight of guns ammunition turrets and maybe strengthening needed for them at the rear.Here's some background - the text which accompanied that photo on FB's 'PlaneHistoria' (I fianlly found it!)
'The incredible power of the German MK 108 30 mm autocannon is shown here on this Blenheim IV light bomber, which was hit by the weapon during British tests. Even the stoutly-constructed and rugged B-17 Flying Fortress was known to disintegrate when hit with as little as four MG 108 rounds, and the more lightly constructed RAF Lancaster and Halifax heavy bombers were even more susceptible to fatal battle damage from this weapon.'
To which I replied:
'can I see some evidence to back up the claim that the Lancaster was more lightly constructed than a B17? I find this slightly difficult to accept without clarification'
Reply
Rod Wylie
plenty of evidence around.
The Lanc was built light so as to carry large bomb loads.
Good read would be Lancaster Men by Peter Rees.
Me
Rod Wylie and yet it has an unladen weight almost the same as a B17 and is slightly smaller in all dimensions...? I'm crying BS
Jon Chapman
it was built differently, using a geodesic tubular steel design which had a very high strength/weight ratio, and was more spacious and could vary a higher volume as well as weight.
Rod Wylie
Or maybe'Big week' by James Holland
Me
Rod Wylie instead of digging yourself in even deeper, find me an extract from either book which refers to the Lancaster being 'lightly built' or more vulnerable to battle damage than contemporary four engined bombers.
Rod Wylie
Jeez , I'm a busy man.
Cant you read them yourself ?
Me
No, because there don't seem to be any [references online]. Someone with your self declared level of expertise and certainty should easily be able to dig those references out, eh?
... y'all know - the usual social media descent into snippy comments and replies (mea culpa!). But in all seriousness, has anyone ever heard anything to support this claim? As others have noted, Lancaster vulnerability would seem to be because of the nature of night fighting and context, NOT the airframe per se. As for 'lightly built', surely that is indeed just boll0c£s?