shot about 30 rounds point blank all over wings and fueslage and didn't do any damage but the bomber was able to shoot a short burst and kill me.
I think this is the key: "all over wings & fuselage".
A B-17 is a tough aircraft. A single 30mm usually doesn't do fatal structural damage when hitting fuselage or wings (though it can wreck single components, like a flap or rupture a fuel tank). And when 30 rounds are "all over", it's not far fetched to assume that a considerable number of them didn't hit anything at all. Unless you hit the cockpit, you need to concentrate your fire with the Mk 108 just like with any other gun - it just takes far less rounds to reach a critical damage level.
The 163 on the other hand is a small and fast target, but not very durable when being hit by a barrage of .50 cal rounds:

(The 163 in the picture got several hits on my bomber, but they had been "all over" too - you can see one of my flaps floating away. I lost the other too, and IIRC one gunner position. Had all those hits been in the same area, I would had lost my bomber.