The ribs of the human body are there to protect vital organs. Heart, obviously being the most important and lungs being the next. Those require major arteries to pump blood into them for them to function.

Your question is very vague, because there's many types of ammunition that come in different calibers, and different calibers of firearms have a variety of different ammunition. That's a big rabbit hole.

Let's assume someone is hit in the chest with a low caliber firearm with low grain bullets that aren't tipped in any special way. That bullet is passing through bone which in turn becomes its own projectile with force exertion towards your most vital organs.

If you throw in the mix, for instance, hollow-point ammunition, its main intention is to fragment when it hits the initial target. This sends many fragments of the bullet into the vital zone hitting more bones that fragment into the vital zone along side the bullet fragments.

That bullet is also-most likely-taking clothing and other foreign objects with it into the internal organs. All of these can cause a lot more complex operations and increase infection.