If "For every action there is an equal and opposite reaction", why does a gun not injure the shooter's hand? If "For every action there is an equal and opposite reaction", why does a gun not injure the shooter's hand?
Would not the "equal and opposite reaction" of firing a bullet be the kickback the shooter feels?  How can the kickback cause (little or) no injury, but the bullet cause great harm?
 A: There is actually a back reaction on the shooters arm. But since the gun is much heavier than the bullet, the velocity of the gun is much smaller.
This follows from momentum conservation, which states that the velocity of the gun is $v_g=v_b\frac{m_b}{m_g}$.
Next the energy of the gun is also absorbed by a larger impact area, than the bullet, lessening the effect even further.
A: It seems like people watch too much TV around here.
Who says there is no injurious recoil?   Watch this, or just go to Youtube and search for "gun recoil".  Mostly you'll find rifle recoil videos, but there are some featuring handguns, such as the second vignette in this one  (The fourth vignette in that last video is quite troubling.)
A: Because the bullet is very small compared to the handle of the gun. The bullet exerts a high pressure (Force per Area) and is able to pierce through skin.
The backreaction of the handle is distributed onto the whole hand.
A: Think of force as the change in momentum with time 
Mathematically formalising this:
$$F=\frac{dp}{dt}$$
$$p=mv$$
By conservation of momentum if the bullet leaves the barrel at some velocity (albeit quite high) comparatively the mass is incredibly small with respect to your hand 
So although the momentum imparted to your hand would be in the opposite direction but of equal magnitude as the bullet
Since the mass of the gun and your hand is so large the change in velocity of the gun and your hand would not be sufficient to injure your hand 
This follows the conservation of momentum 
$$v_{g+h}=\frac{v_{b} m_{b}}{m_{g+h}}$$

I dare not say the change in velocity of your hand is small as you will note the recoil one experiences when he fires a gun is fairly significant and enough to throw an inexperienced gunner off their feet I.E when shooting a shotgun 
A: There are several reasons for this:


*

*The momentum of the bullet and the momentum of the gun are distributed over different area. The recoil distributed over all the area of the gun's butt, while the bullet's momentum is applied to a much smaller area. Therefore momentum per unit square is much less in the case of the gun. 

*You need to include also the momentum of the gases that push the bullet. They carry away some of the momentum and thus can make the recoil less harmful. 

*In all modern guns there are special systems that damp the recoil by increasing the time at which it is transferred to the shooter or by dissipating a part of it.
