well consider how many la-7 with 3 guns where available IRL....
A bad thing for the allied bomber crews was they did not knew that it was rockets killing them according to the book "Luftwaffe over Germany".
The rockets where not seen until they hit, and allied crew thought they where being killed by ground-ack.
That might have been true in some cases. I read (can't remember the name of the book) that one B17 saw the rockets being fired at it (must have been the tail gunner reporting it to the pilot), and tried to evade the rockets. While doing this, it collided with another B17. So without any rockets hitting, two B17s were lost.
Here is a video of a Bf-110 that has used 21cm rockets to attack a B17. They are the black puffs of smoke the B17 is surrounded by.
http://www.youtube.com/watch?v=ljnBwoIgmJg The 110 continues on and attacks the B17 with cannons.