When I was researching the 15th AF history, I found a site with part of a Lufwaffe ordnance report describing how many 20 and 30mm hits (randomly spread) could down a Viermot. AS I RECALL the figure was twenty for 20mm and perhaps five for 30 mike-mike. There are a few online references to 20mms but now of course I cannot find the document and would welcome referral to any source.