When you have one "in the chamber" with the rifle uncocked, the firing pin is exposed at the bolt face and is touching the primer. It is kinda pushing the cartridge forward a bit. IF the rifle is dropped on its butt hard, The cartridge acts like a person in a car without a seat belt. The rifle and pin suddenly stop, but the cartridge keeps moving until an outside force stops it. The outside force is firing pin. It is possible for the primer to detonate at that point. Lots of things have to be right BUT that is why some six shooters have a bar that comes up and rest between the hammer and the cartridge when the hammer is down and the pistol is not cocked. If the hammer strikes the ground at the right angle the gun could go off.