Firing a gun now throws a real flash of light from the barrel. It lights the ground, the walls and whatever is standing in front of you, so shooting at night gives your position away in a way it did not before.
Each shot kicks up a small puff of smoke.
The flash sits on the barrel itself and tracks it through the weapon's recoil, in every aiming direction.
Shots come from the barrel
Bullets now leave the end of the gun. They used to spawn from a fixed point above your character regardless of which way the weapon was pointing.
Shotgun pellets come from the barrel too.
This is most noticeable when firing left, or up, or while stood close to cover: rounds now start where the weapon actually is, so what you can and cannot shoot past matches what you are looking at.