yeah, i've been playing this sim for over 10 years now, and there's no way i could take out a whole formation of bombers, not without a whole lotta luck, and even then, it ain't gonna be easy.
if i play with realistic gunnery turned off, sure, you can take off an entire B-17 wing with one round from MG's, never mind the cannons! also sounds to me like you've got realistic gunnery turned off, else you're a natural-born uber ace, and should start giving gunnery tips to the whole community...
with realistic gunnery, you should be going more along the lines of: 'wtf? i'm emptying all my ammo into this bastard, and he just won't go down!'