Dingy,
The ShVAKs aren't low velocity at all. The effect is caused by the supercharger air intake on top of the cowling. It cuts off a lot of the view on the bottom that most planes usually have, making it seem that the rounds are dropping off real bad. I tested this a while ago, maybe back in beta. Did a comparison with a 109G-6's MG/151, and found that the ballistic paths were very similar, using strafing passes along the ground and marking the paths of the tracers under specific G's. Basically, it's an optical illusion, caused by the intake.