The trick with those MG-s is to focus all your firepower into a single part of the enemy plane.
Granted this is difficult because they are slow and wing-mounted so your aim can be off.
Also if you face an enemy with a brain he will not stand in one place and fly in a straight line for you either.
You need good range alignment of your guns (about 300-400meters is great), and even then you might have to aim with only "one" of your wings or you miss entirely.
On planes like the hurricane there's like what, 12 of them? If you hit a single part with all of those, it adds up and becomes devastating, just raw firepower alone.