In the realworld, ingame, or what?
Seriously, though - like RL, they let you have highly visible shots every so often for ease of aiming or aim adjustment, without needing to make every bullet really really obvious. This is mostly how they're used by the stock weapons.
Of course, you can also do fun stuff, like have a gun that shoots two different kinds of ammo alternatingly... for example, a magazine that alternates between armour-piercing and incendiary rounds with every shot, or a launcher that fires two bouncy grenades followed by an impact-grenade, then two more bouncy grenades, then another impact grenade, and so on and so forth.