Let's correct your statement in-line:
liokault wrote:If you are an astronaut, you move backwards with as much force as you apply forwards.
This is true, by Newton's third law. However there should be clarification here. Force = mass * acceleration. The gun (which in our case is taken as "part of" the human body) exerts a force on the bullet via the controlled explosion of gunpowder. The force exerted on the bullet creates a change in momentum from 0 to +10 kg*m/s (forward momentum). That same change in momentum (remember, Force = a change in momentum), is then exerted on the human body via the gun grip. As stated previously, assuming a body+gun mass of 100kg, this means, the human body will experience the same change in momentum backward (-10 kg*m/s). If you shot that gun on a frictionless surface (eg you are sitting on a sled floating on an enormous air hockey table), you would then be moving backwards at 0.1m/s.
The only reason why you do not move backwards is because of friction between you and the earth, and the muscles in your body maintaining your frame, and transmitting the force from the gun to your feet. If you somehow let your legs and abs go limp when you shot the gun, you would fall over backwards.
This is the same as on earth, except on earth, your joints move and absorb the force, meaning that you don’t float slowly backwards.
This is partially true. your muscles push back at the force to maintain a frame, and allow the force to be transmitted into the ground via friction. If there were no friction, you would float slowly backwards.
Your joints themselves to not "absorb" the energy, although you may feel like you do. Your joints do absorb SOME of the energy, but again, only a slight amount to heat in your joint lubrication.
By your rational, anyone shooting a gun would themselves take the damage of a bullet in the hand every time they pulled the trigger, the principle is the same.
This is untrue, like I said before. The force exerted on gun by the explosion is spread throughout the gun grip. On the bullet, the same force is focused onto a single point, be your bullet a ball or a typical conic type shape.