I think I can help you, Yvonne. My hubby was in the Army and is an engineer as well, and was able to explain the mechanism of a handgun to me. This information is quoted exactly from him. He tailored it to the fact that the handgun Lois is holding in the episode is a semiautomatic.
When you pull the trigger of a gun, a leverage mechanism raises the hammer. At a certain amount of pull, the leverage mechanism drops off of the hammer and the hammer is released.
When the hammer is released, a spring propels the hammer forward. In modern firearms, the hammer hits the firing pin (which is a part of the gun).
The firing pin is pressed forward at high speed and strikes the primer cap on the end of the metallic cartridge case. The primer cap is filled with a very sensitive explosive (probably fulminate of mercury, unless they've changed it to something else in the last few years). The fulminate of mercury explodes, is vented through a small hole into the main powder charge, causing it to ignite, which causes the bullet to be forced out the barrel. The blow-back mechanism causes the slide to move backwards and eject the cartridge, which then is thrown out of the right side of the gun and falls to the ground.
I hope this information is helpful.
Nan
