So I understand that when you fire an automatic rifle the gas sends the working parts rearward to pick up the next bullet, but when the last round is fired the working parts are fully rear and the hold open recess stays open. Why does it do this?
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I’ll explain it for pistols since pistols are what comes to mind with the slide staying locked back after the last round is fired.
You are correct, when you fire a (semiauto) pistol, the slide gets knocked back as each round is fired, then comes forward and engages the next round. However, below the last round in a magazine there is a little plastic block called a “follower.”
After the last round is fired, the follower reaches the top of the mag, and a little tab on the follower sticks out the top. This tab engages/trips a lever next to the chamber and this lever prevents the slide from coming forward, and essentially locks it in the rearward chamber-open position. It may serve other purposes, but the main purpose is to provide a visual/physical cue that the mag is empty
There a latch system near the back that’s depressed by the next cartridge in the magazine. As long as there are cartridges in the magazine, the latch can’t grab the bolt (the part that slide back).
Once the magazine is empty there’s nothing to hold the latching mechanism down, and it grabs the bolt on the next recoil, holding it open until a new round is chambered.
Edit:typo
There is an excellent (https://www.youtube.com/watch?v=NZX7X3pJgH8) from the US army that goes through all the parts of a small arms action and how they work, step by step.
It’s normally done with a latch that catches a bit of the magazine that pushes the bullets up, but doesn’t catch on the bullets (because they are round and the catch is located in a position they can’t easily be within the magazine).
First remember you have a magazine full of bullets in the gun. This magazine is a box, and a spring that pushes up a follower, which is the little plate the bullets sit on.
Say you’re on the last bullet. The bolt comes back, goes forward, strips a bullet, fires. But now there’s no bullet in the magazine, the follower is at the top position, and it pushes against a latch. Now the bolt comes back, goes forward, and catches against that latch, holding it open.
It depends on the firearm. In the simplest scenario, nothing stops the bolt from returning forward.
In the next simplest scenario, the follower in the magazine (what pushes against the cartridges) extends into the receiver in the path of the bolt and blocks it from returning. This basically only lets the user know that they’ve emptied the magazine, since removing it will let the bolt return forward.
The more typical and useful scenario is a firearm and magazine combination that have a bolt hold-open. Typically, this works by having a tab on the follower of the magazine that activates a part to hold the bolt back. Since it’s a part within the firearm itself, you can remove the magazine and the bolt will remain held back. This allows a user to insert a fresh magazine and deactivate the bolt hold-open without having to completely recock the firearm. Typically the hold-open is engaged just with friction, so either manually forcing it back out with an external bolt release control (typically a button, slide, or lever on the receiver) or charging the weapon (e.g. with the slide on a handgun) will disengage the hold-open and allow the bolt to return forward again.
Others have answered, but I just want to point out that not all self-loading firearms are gas-operated and not all self-loading firearms have a last round bolt hold-open (LRBHO) or slide-lock on empty.
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