docjonel, when you press R with your shotgun you are not reloading. When you fire with shotgun, you need to press R to remove the empty shell you just fired from your gun. You reload the gun when you insert a shell on the shotgun. You always need to press R after shoot with a shotgun or mosin to get next bullet. But if you press R and the bullet wasnt fired, you just take that bullet off your gun.
Think the bullets from a shotgun as a stack, when you press R you are saying, next bullet, please.