Which part of the firearm loads, fires, and ejects shells or cartridges?

If your possible answers are

stock

action

barrel

trigger

Then the correct answer is action. The action part activates when you pull the trigger. What happens is that a little metal piece hits on the back of the bullet and ignites it, and it gets propelled outside. The weapon recoils and the casing falls out, and the next bullet is put into the barell.