system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If every action has an equal and opposite reaction, why can a bullet do tremendous damage to someone, while the recoil from the gun cannot?

Think about this, a large bullet that hits someone in the upper arm can dislocate the person's shoulder, but the recoil cannot. Sure, the bullet focuses the energy onto a smaller area, but is that alone the reason? That doesn't make sense to me, because incredibly large guns (for instance, a anti-material rifle firing a .50 BMG round) can take an arm or off even when they hit a good distance away from the point where the limb is torn off, but the rifle's recoil is enough to be managed without injury. Similarly, some bullets can knock a soldier unconscious if they hit them in the helmet (dispersing the energy over a large area), but the rifle doesn't damage the shoulder, despite the fact that A) the but of the rifle has less surface area than the inside of a helmet and B) the shoulder doesn't take that much to be dislocated. 

Does it have something to do with the difference between force, energy, and momentum? Or the difference in velocities? Really confused here.
gpt
A bullet exerts a ton of pressure, much more than the gun does when it recoils.

Pressure = momentum / surface area / impact time.

Momentum is the same for the bullet and the gun, but the surface area and impact time vary drastically. The surface area of a gun is the area of the grip, while in the bullet's case it is less than a square centimeter.

But much more importantly, is the impact time. As the bullet accelerates along the length of the barrel, a continuous force is applied to the gun. Additionally, because the gun is much more massive, this force goes into accelerating the gun, which acts as a buffer when translating momentum from the gun to the hand.

When a bullet hits someone, travelling at hundreds of meters per second, the time of impact is tiny, resulting in a huge pressure. Even if the bullet was the size of your head, the time of impact is still so small that the pressure felt would be massive. This is why getting shot in a helmet can cause concussion or death.