That's one of those problems that I'll say is a problem if it ever actually happens. Generally speaking, you're just going to end up using a lot of ammo to achieve suppression and not much else. There will be outliers, but having good line of sight and good visibility and no cover and so on at 300m+ is pretty unlikely.
If it still bothers you, there is of course another way to see it: the shooting action is not a single long pull of the trigger, but rather the total result of all of the fire over 10-15 seconds of time. Maybe most of the rounds missed by a mile but the last grouping was right on target. Maybe the first shot hit but then 15 missed before another one or two found the mark. As always it's the job of the players and GM to try to bring these details to life.
@andresk - seems likely that whatever house rule you're referring to came from before the revisions in the beta, which rebalanced ammo dice to some degree.