Firing a gun that has a bullet 10 times as massive as the gun would be a poor idea for several reasons. First, the recoil from firing such a large bullet would be enormous, potentially causing serious injury to the person firing the gun. Second, the accuracy of the shot would be very poor, due to the immense amount of force required to propel the bullet forward. Finally, there is a risk that the gun itself could explode if fired with too much force, again causing serious injury or even death.