Yes, it is. HiTech has explicitly stated so. This is one of the reasons that fighters attacking bombers from dead six feel like the bomber's guns are more powerful. The rounds fired by the attacking P-51 are moving faster and thus have higher parasitic drag, slowing them down faster and the bomber is running away from them. The rounds fired by the B-17's tail gun are moving slower thus having less parasitic drag thus keeping their energy better and the P--51 is running into them.