Large-scale combat - whether in distances/ranges or in quantities - can get tricky. IMNSHO, the main thing that you should focus on is a) where the characters are now, b) where most/all of them can get to in the next turn, c) who's likely to be shooting at them in the next go-round, and d) who else is available for the opposition to shoot at. Know the movement distances your PCs can take; be able to say whether or not there's cover within easy, tough, or 'you better roll' reach of their current cover. Decide - based on their cover, their movement, and who else is available as a target (people are more likely to engage a nearby target under serious cover than a very distant target under negligible or no cover) - who is shooting at them, and toss dice only for those NPCs. And if NPC Bad Guy A is going to be shooting at NPC Good Guy B, chuck some dice for show but don't worry about it - whatever happens is what fits into the story.
Also keep in mind what the player goals are for the fight. Sometimes it's easy, really; if the players need to get from Point A to Point Z without getting killed, and aren't going to need to kill every single other MFer on the battlefield, then you only need to roll 'X guys are shooting at you this phase'. Sometimes, though, killing every other MFer is the point, and then you should focus on 'engagement zones' - the areas within which any particular NPC is going to automatically shoot at targets. And even then, it can still be relatively easy ...