All 3rd person "shooting" games I know (wich work and are actually fun) have an automatic AIM.
So that the gun is locked within a certain angle to the target.
The player just needs to point close enough to the enemy.

If you want a totally free shooting-behavior (the player
shoots exacly where the crosshair is pointing to)
then rather use a first person view.

Using a precise AIM in 3rd person is no fun.
Its the problem of the geometric offset between
the vector of the gun and the camera position. Its simly much less
natural than first person shooting.
Thats why the gun should lock to the target, if its
pointing to it within a certain angle.


In third person you also need to have decent animations
for the player to look good, as the player character is seen
all the time.

Apart from that, its very important to make a camera that
works smooth, especially in indoor areas.

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On the other Hand, 3rd person has advanges, as you can work with lower resolution textures and lower detail objects,
when you keep the camera at a certain Hight.
In first person, the player can much easier loose the
illusion of a well crafted gameworld, as everything
is seen much closer.

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But: making a first person view is much easier to do than correctly implementing
a 3rd person view.
In first person you basically just need to move the
camera, and shoot to the "middle".

In 3rd person you need a rather complex follow camera, better animations
and a good aiming precedure.