Crossbows and bows and ranged spells work well enough in a fantasy game. But in a modern or sci-fi game where guns are available, it feels off to dump round after round into some guy doing chip damage until he falls over. Especially if both sides are using guns, and are standing in the open. The +2 from cover isn't enough to risk being in sub optimal range or risk flanking moves. The lack of suppression also makes flanking unfeasible unless you're really fast.
A gun isn't a crossbow where you're shooting 1 bolt every 5 seconds. Either you're lining up a shot, or dumping a magazine. Usually something in between. I can forgive the abstraction for gunslinger classes because they're usually using black powder weapons, lever action, or revolvers at best.
I've considered rules like automatic weapons have advantage against people in the open. Or maybe doing tripe damage (3d8 instead of d

. Ammo, if tracked, should also be measured in bursts. Maybe give weapons a suppression or overwatch ability to get a free shot on a guy if he moves.
Just something other than doing a find-replace "crossbow" for "rifle".
That's good, because there aren't any.
But I shouldn't have to. That's like giving someone a dictionary and claiming it contains every story ever written.