I can't seem to find any rules regarding how one would shoot at a horde that was engaged in hand to hand combat. Say you have a horde magnitude 40, you have 40-50 cultists in the horde. One marine gets engaged in close combat on the edge of the horde. The devestator still wants to help clear the horde out, so he opens fire full auto into the mess of 50 people. Now, not ALL 50 people are attacking the one marine in close combat, so there are going to be some just hanging about.
Does the devestator still suffer the -20 to shoot into hand to hand if over half the horde is not engaged? Does it depend on horde size? And does the remainder of the horde get to shoot as well as beat people in close combat? Maybe use the 10s diget of the horde's magnitude to determine how many people have to be engaged for the horde to be considered 'pinned' in close combat (so a magnitude 25 horde could be shot at and shoot as a magnitude 10 horde if ONE SM was in close combat, but if TWO sms were in close combat it could not shoot, but SMs outside of CC would suffer -20 to hit? <rant> Then that bears the question, why does shooting a bolter on full auto increase your chances of NOT hitting your friend who is covered with enemies </rant>?)
A page pointer would be most helpful, or your opinions on the matter.