The speed of a typical rifle bullet is somewhere around 1000m/s. Which means you get 16 meter per frame, realistically.
So, that's quite fast, no slow moving bullets. Most likely you'll just implement hitscan instead of real bullets.
Anyhow, how to deal with it? Well, a red line "sniper warning" works in a lot of games. Inaccuracies in firing also works (get closer and you get more damage on average). But in almost any game with hitscan weapons you want cover or you'll get lead poison.
I just might do that.
Give the illusion of realistic gunfire that doesn't exist mechanically - have enemies shoot, but miss. This way, enemies can "realistically" start shooting fast, lethal bullets at you without actually killing you. It will naturally give the impression they can't aim very well, though. Make it work on a timer; an enemy is required to shoot at you for X seconds before they score a hit. You can even have hit points and have every bullet chip away at them, without showing this information and presenting the bullets as missing visually. So "hit points" is more like "luck"; stand long enough in gunfire, your luck runs out and you die, while giving the impression that only the last bullet hit, and with lethal force.
I get what you're saying. Sounds like a good idea.
If you want the gunfire to be realistic, then you shouldn't be able to see the bullets. Have gunshot sounds and dust flying off obstacles/edge of screen where the bullet hits, unless it hits you. Maybe make the path of the bullet slightly visible like a thin, fading, white line.
have enemies shoot, but miss.
this and auto-cover-animations that show why the enemy is missing. Maybe you can introduce shouts and stuff to warn the player. Muzzleflash-symbols at the edge of the display area with an auto-cover reaction for sniper shots might help too. For machine gun fire you can visualize tracer ammunition.
What's auto cover. And I could use all the "warning: incoming bullets from offscreen" suggestions I can get. Thanks.
By 2D do you mean top down or do you mean side view? Not that it really makes a difference.
I'd follow the Metal Gear Solids and have enemies aim (giving you time to get out of the way), possibly with laser guides, and then the shots are instantaneous raycasts.
Side view. Laser sights now seem very likely.