Let’s take a look inside a gun as it goes off. For the purposes of this, we’ll use the simple Blowback action of a Sig 230/232. It uses a .380 ACP, otherwise known as a 9x17mm, or 9mm kurtz.
First we slide a loaded magazine into the grip of the pistol.
Once the magazine is in place, we pull back on the slide. This has the single effect of cocking the hammer.
Now, we release the slide. The spring around the barrel will push the slide back into place. As it moves forward, the slide will strip one round off the top of the magazine and slide it into the chamber. The spring also holds the breech face, which is part of the slide, against the back of the casing. The weapon is loaded and ready to fire.
We pull the trigger. The sear rotates in the grip and the hammer is released. It falls forward to hit the back of the firing pin. The force of this will drive the pin forward until the end of it pops out of the breech face and strikes the primer in the case.
The primer ignites and sends a jet of flame through the flash hole in the back of the case and into the powder. The powder ignites, and as it burns, it gives off gas.
Gas takes up a lot more space than solids, so as the powder is converted from solid to gas, the pressure in the case begins to build. If this were a firecracker, it would explode out in all directions, but it can’t. The chamber holds the sides in place, and the breech face holds the back. The only way for the pressure to escape is by pushing the bullet out of the way.
The only place for the bullet to go is down the barrel. In a space of 2.5 inches, the bullet accelerated from a dead stop to over 1000 feet per second. Some of the powder is still burning and comes out of the front of the gun as fire. Some of the powder didn’t get burned at all. Most of it coats the inside of the barrel as a thin layer of soot.
Now. At the instant the bullet began to move forward, that same pressure caused the casing to move back. It had to push against the breech face, which is part of the slide, and held in place by a spring, so it didn’t move as far in that amount of time, but it’s on it’s way.
On one side of the breech face is a little hook that is just big enough to catch the rim of the casing as it gets blown back. Just before the slide reaches as far back as it can go, a small rod on the frame of the gun pokes through the breech face and pokes the side of the casing opposite the little hook. This happens just after the casing has been pulled completely out of the chamber, and will cause the spent shell to be ejected from the gun.
Part of the process to this point is pushing the hammer back and cocking it in place.
When the slide is as far back as it can go, it still has a lot of energy to bleed off. It transfers this to the frame of the gun, which causes the entire thing to move. This movement is usually a rotation that makes the barrel of the gun climb up. This is due to the force being applied to from above the center of mass of the gun, and the way the shooter’s wrist works.
When the slide has come to a complete stop, it’s entirely possible that the gun itself is still kicking backward. She slide doesn’t care, and the spring pushes it back to the closed position, picking up the next round as it does so. The gun is now ready for a second shot.
In other semi auto’s the process is similar. In the .45 Auto, there is additional movement as the barrel cams downward slightly, then locks back into position using grooves cut into the top of the slide, but that doesn’t matter a whole lot for this topic.
A second type of semi-auto operation is called Gas Operated. This is far more common in rifles, but the Desert Eagle is notable for using it.
In a gas operated action, there is a small tube near the front of the barrel. A small portion of the gas which is driving the bullet forward will pass up this tube and move back to the bolt of the gun. Pressure from this gas will drive the bolt back. This ensures a slight delay and allows for the bulk of the pressure to be out of the barrel before the breech is opened. This allows for more powerful rounds to be used in an automatic design.
One last bit to note about recoil. It’s fast. Unimaginably fast. Still, there are people that think they can stop a rifle or shotgun from reaching their shoulder if they just hold it an inch or two away. They can’t, and there will be strong language, and possibly tears, when they try.