You are talking about a Moen cartridge. You have to loosen the cartridge by turning it 90 degrees left and right. For more info, check www.moen.com. Shut off the water supply remove the clip Rotate the cartridge then pull it out get the same replacement cartridge grease the new cartridge slide it into place install the clip turn the water back on if the hot and cold are reversed turn the stem 180 degrees Its that simple

The handle must be removed to allow access to the retainer clip, then the cartridge pulls out. You may need a cartridge puller if the faucet is old.

on the bottom of the gun behind where you clip on the tripod

First, the bullet is probably not the word you are looking for. You probably mean cartridges. The bullet is just the very end of the cartridge.

You also probably mean magazine, not clip.

An 18 cartridge magazine holds 18 cartridges.

This term often refers to a bullet cartridge inside of a magazine (or commonly referred to as a "clip").