I agree with Grun and Wot.
When playing IL-2 or Forgotten battles, the spectacualr visual effects of smoke, shrapnel and explosions really give you the illusion that you are smoking your target pretty bad. However, in many occasions, when you really go over just how much rounds you hit him with, it's surprisingly low.
My opinion is also that gunnery is the true factor. I'm an average pilot, and I know I don't have the skills to give concentrated bursts on a single area of a bomber. In my case, when attacking bombers, it often takes many bursts and shots which spread usually all over the fuselage and wings. Naturally, it takes a lot longer for me to take a bomber out, than some of the better pilots.