It has less guns, but the nose guns are the same. The wing guns on the P-39D have a lot of bullets but their overall effectiveness is very low. You need to be less than 250 yards to do any real damage, and with the P-39D being rather slow this doesn't happen often.
It's a fair tradeoff, as I'm sure most of your kills are done by the nose .50cals anyways (not counting, naturally, the 37mm itself).
EDIT: It also removes hundreds of pounds from each wing, making you roll and loop much better
EDIT2: Also, the Q has gunpods. While they slow you down a bit and weigh you down a bit, that 1x .50cal is still 50% more powerful than the 2x .30cal that were in the 39D's wing. Each .50cal bullet is about 3x stronger than each .30cal bullet, but on top of that they retain their "punch" much further out and are better long-range guns. Taking the gunpods got most of my kills back when I couldn't kill with the 37mm.