Whenever you shoot a gun you have to be aware of your surrounding and what the gun will do after it is fired. Before you shoot it you should know your target and beyond. Also for saftey you should have eye and ear protection, because the gun is loud when fired and it could kick back. The right shooting position could can be the difference between an injury or you just having another day at the range.
For Example, if you have a 12 guage shot gun with a slug barrel on it this could potentialy have a alot of recoil to it. This is because this is a shorter barrel and designed for taking down big game. When the gun is shot the bullet is travelling around 1500 to 2000 feet per second which is really fast. So by the bullet going forward there will be a force that has to push back on something, and that something is the person firing the gun. This is why you need to have the right stance and shoulder the gun correctly, so that when you can absorb the recoil when the gun is fired, and most of the recoil will go through you and into the ground.
Also, momentum of a gun can play a roll in the recoil that it will have. When it is fired there is usally a shift in momentum because the gun was rest and then when fired it moves forward, then something has to bring it to a stop and that is the person. This is because the perosn is usally stationary and in a way connected to the gun when it is fired, so this is also helps brings it to a rest.