what causes a bullet or projectile to travel down the barrel of a firearm and exit at the muzzle?; what is one thing that almost always happens when a cartridge is fired; what can affect the performance of a cartridge; firing mechanism of a gun; how is a single-shot different from a repeating firearm; which type of action operates automatically each time you pull the trigger; shot pellets and a bullet are examples of which basic component; what happens when you pull the trigger on a gun