Simplify the following text for a Grade 7 native English-speaking person.
Original: All differ in the way the bolt fits into the receiver , how the bolt rotates as it is being operated , the number of locking lugs holding the bolt in place as the gun is fired , and whether the action is cocked on the opening of the bolt ( as in the Mauser system ) or the closing of the bolt ( as in the Lee-Enfield system ) .
Simplified: After firing the gun , opening the bolt takes out the empty cartridge , and closing the bolt puts in a new one .