They knew those issues and didn't try for quad 20mm or such as a stop cap measure?
The prevailing theory in the Luftwaffe by that point was that 30mm = insta-kill on a bomber. A brief burst of fire, maybe two or three rounds per barrel, and you could blow a B-24 apart- then move on to the next one. So, 30mm was the caliber preferred for planes intended to go bomber-hunting. That it had the muzzle velocity of a squirrel throwing an acorn was unfortunate, but how hard can it be to hit a huge bomber flying slowly in a straight line? (Answer: Not nearly as easy as it seemed, apparently)
A far more useful idea, as it turned out- at least in theory- was the BK 5 mounted in a few Me-262s, a 50mm cannon (it saw service on a few other types as well here and there). It had a few drawbacks- low ammo carriage (21 shots), jam-prone, and with a ferocious muzzle flash that tended to blind pilots if fired at night. But, a hit to even the toughest bomber would blow it apart, and with good accuracy and range it was a surprisingly effective tool.
