The simple, universal way to perform combat reloading.
Johnston
Shoot shoot shoot! No more bangbang come out of barrel! Look at the empty place where hot thingies come out! Is it open and bolty thingy locked back? If yes: go on and perform reload procedure. If no: perform IA procedure. Hit/touch/stroke/caress the thing you need to use to release the magazine! throw the empty magazine far far away! get one fully loaded magazine from wherever you put them! slap/insert/ease in the magazine into the hole where your supposed to put them! Pull the handle you pull usually! Continue shooting! End.
Really, the simple answer is because.... The bullets are housed in a container called "MAGAZINE". The bullets are inserted on top of a thing called "Magazine follower" The "magazine follower" is on top of a "spring". When the bullets are shot finished, the "magazine follower" comes up and prevents the "bolt carrier" from going forward into a condition known as "battery" which means "able to fire". So to reset the "bolt carrier" into "battery" you take out the "magazine" and insert a new one. Just pull the "cocking handle" and the "bolt carrier" will go forward allowing your "bullet" to go into the "Chamber". And your weapon wil be in "battery" again.
When your weapon is in "battery", you can pull a thing called "trigger" which will enable your weapon to "fire".