I like to keep shooting into combat simple (imagine that). I prefer to use a percentage method. Sure, you MEANT to hit the bad guy in the chest, but you ended up hitting your friend in the back of the head. Shooting into combat should always be a risky proposition.
Divide 100% as evenly as possible with all of the foes and allies in the area in which you are firing into.
1 Ally 1 Foe: Ally 1-50%, Foe 51-100%
1 Ally, 2 Foes: Ally 1-33%, Foe1 34%-66, Foe2 67-100%
2 Allies, 2 Foes: Ally1 1-25%, Ally2 26-50%, Foe1 51-75%, Foe2 76-100%
etc.
Roll a D100 and see where your risky shot attempts to hit.