Question

why is it advisable to hold a gun tight to one's shoulder when it is being fired?

Answer 1

Because its recoil may hurt you .,. Recoil means , the gun will exert force to the person who is holding the gun ......., this happens due to Newton's law ,that is , EVERY ACTION HAS AN EQUAL AND OPPOSITE REACTION

Answer 2

As we know that every action has equal and opposite reaction ,the force exerted or pressure cause to release a bullet from gun is applied equally on the body of person carrying it.
So to cause less harm to person using highly advanced gun ,they are advised to place gun on shoulder to get minimum injury.