Just from what I've seen from the various videos and such, it seems like both types of weapons are available (ie: "instant hit" projectiles and "moving" projectiles).
But since you don't have any force powers and we have no word yet that you'll face anyone in combat who does, you shouldn't have to worry much about people "dodging" your bullets, blocking them and pushing them back in your face. ; )
Though looks can be somewhat deciving. After all, in Unreal we've had a gun for years (UT's Shock Rifle and it's many varients before and after) that fires projectiles that appear not to be instant hit, but really are, as well as projectiles that "move" that can be dodged (even though it's not very easy to do so).
The ammo thing I wonder about. Since you're not fighting other commandos, how will you get more ammo? Will there be ammo drops supplied to you by the Republic? Is any alien weapon ammo compatible with your's?
Or will you be forced to switch to alien weapons when you run out of your standard issue?
Or perhaps they just give you so much ammo that it's more than enough for any mission.
Otherwise I suppose you'll be left with your little melee blade to fight! ; )