It seems like a lot of people don't understand how cartridges work.
When the gun is fired, the case creates a seal between the cartridge and the barrel. The force of the explosion fires the projectile out of the gun.
Without the rim if the gun were to go off this would still happen but the rim, increases friction with the face. Without it there is a very real danger that the barrels may shoot off the gun in the direction pointed. because of the great force exerted they would almost certainly leave our atmosphere and continue until they collided with the moon or another celestial body.
Einstein stated that for every action there is an equal and opposite reaction. The person firing the gun, having more mass than the barrels would be driven into the ground but luckily wouldn't travel as far as the barrels. The most likely scenario would be that they would come to rest less than a mile underground.
It is thought that this is what happened to Lord Lucan who was riding Shergar when he took a potshot at a lofty starling.