Good luck hitting with your minimum -6 on that roll for Extreme range, and I would quite frankly give you Strong winds to deal with as well on a grenade traveling half a kilometer, so that's -10. So even if you went Agility 9 Heavy Weapons 6 at chargen, you're trying to get 3 hits on 5 dice.
Take Aim with Vision Magnification makes that -3 for Range, and quite frankly, saying "Strong winds" at 500m (a third of the distance snipers are capable of) just reeks of GM cheating, unless you make a point beforehand of mentioning there's a storm going on and everyone else also gets the penalty.
So all in all, you are far more likely to have a -3 than a -10, which means a Professional Rating 5 grunt with an assumed skill level 6 in Heavy Weapons has a 70.09% of nailing the shot, 19.51% of getting 3d6-2m scatter (so 8.5m on average), and 8.67% of 3d6-1m scatter (so 9.5m on average). Add in a wireless smartlink for +1 die, and those numbers become 76.59%, 15.90% and 6.36%, even without further Take Aim actions.
With Professional Rating 4 grunts and assuming skill level 5, you're less likely to get in trouble from that distance (42.94% of nailing the shot), but at 150m, Take Aim with Vision Magnification turns that -3 into a -1 from Range, so if there's no/light rain/fog/smoke, full/partial light, and no/light winds, that's 62.28% of nailing the shot, even without further Take Aim actions. And
that is what I'm most concerned about: grunts being able to wipe out the party with a single not-unlucky shot.