Well, actually the BK 3,7 was very powerful. Original round was able to penetrate 58mm armor, more than enough vs T-34/76s (the slope of its armor had little effect), and also vs T-34/85s, except turret sides. Later it received another round which was able to penetrate as much as 120mm!
http://www.deutscheluftwaffe.de/arch...dkanone37.html