We have decided to TRY (but not officially adopt) a less random formula. The player shooting and all players in the close-combat make an Initiative check (d6+I). The shooter may target any one of the units that he beat. The shooting is resolved normally, with the except that the defending unit has a cover save as if blocked by another unit (4+). If all the units in close-combat made higher Initiative checks, than the shooting unit was unable to get a shot.
Our flavor for this is simply that even in aggressive close-combat, the models know what is going on across the battlefield (how else would we justify moving our units the way we do in response to our enemy's movements far beyond line of sight?) and are aware they are being aimed at. The fastest responding units in close-combat are able to make themselves practically invisible by moving unpredictably between within the melee, and agile tactics allow them to put the enemy between the unit shooting and themselves.
Now, here comes the interesting part: not only can the person shooting fail to beat ANY Initiative, and therefore simple be unable to fire effectively at any target (maybe they fire but miss their target, maybe they hold because they can't effectively track their target, but for whatever reason a failed Initiative check means they were too slow to deal with the chaos of close-combat), but IF is able to target a unit, it may NOT be the unit he wanted to hit. If my enemies are Eldar and Tyranid, and they are locked in close-combat, I may want to weaken the Eldar, hoping to even the odds of the close-combat and thereby prolonging it, or at least increasing the overall body count. But if only the Tyranid units scored lower than my Init check, my firing unit is unable to get an effective shot on any Eldar. I may want to chose to not fire at all. After all, if I kill the Tyranids, the Eldar are no longer locked in CC and might find some of my targets.
The second amendment to this rule we are also going to test out is if, under these circumstances, the shooter deals 25% fatalities to the unit it will take it's Leadership roll as normal to see if it flees. But under the case of a failure, it will fall-back as if falling back from close-combat. Meaning: the opposing force in CC gets the sweeping advance, not the unit who did the shooting. We do this to add another bit of risk to firing into CC. Even if it's none of your own units, you may be strengthening one of your enemies a bit too much by granting them a sweeping advance (meaning victory points if your counting) or a saving some of their units from a death you'd rather they would have had.
So, the steps are:
Shooter picks the target CC, then verifies range.
All units, including shooter and all units in CC, make d6+I checks.
The shooting unit may choose any unit he equaled or beat to fire at, with the shooting otherwise resolved normally. (the defender has a cover bonus of 4+ granted by the close-combat)
If this attack causes a unit to fall-back due to losses, it is treated as if they are falling back from the CC, not the shooter, and the attackers in CC may make a sweeping advance, if possible.
That's what we'll playtest this weekend, since we're often playing free-for-all with 3 armies or more.