Firing Rates Explained

Fully Automatic, Selective Fire, and Semi-Automatic

Reading Time: 10 min

The more “rapid” the fire that is wanted from a gun, and the more sustainable and effective the shooting needs to be, the more the gun will need to be purpose-built to support it, and the more advantage there is in military-optimized rifles. This is often missed or hidden component of the risk potential of civilian assault rifles, particularly in the hands of mass shooters.

Functionally, both are capable of rapid-fire, one is just usually a bit more rapid than the other, and they are far more similar than not. The design features that distinguish these two modes are worth understanding — just a little bit —  because they are the only things that differentiate prohibited military assault rifles from the civilian versions, and the bare fact that the difference is very small matters. A lot.

These firing modes are are rooted in mechanical differences which we will see in a second are literally quite small, but in application there is significant overlap, and both firing modes can exist in the same gun, and often do. The difference really comes down to just one thing: how the gun handles repeated shots.

Both types of guns can be loaded with more than 1 bullet at a time (which come from the magazine). The bullet gets loaded into the firing chamber, and then the gun is cocked so that pulling the trigger will release the firing pin and initiate the explosion that fires the projectile part of the cartridge.

With any gun, once a bullet is fired there is a process that has to happen before the next shot can be fired:

1. Trigger is pullled.

2. Firing mechanism is engaged to fire the loaded (“chambered”) bullet.

3. Eject the spent shell casing (the “leftover” of the cartridge).

4. Load another bullet into the chamber (from the magazine).

5. Cock the weapon to prepare to fire again.

In older, and many other styles of modern firearms too, each of these steps is a manual process.

What makes a gun “automatic” is that this process is done “by” the gun, and it starts with the first trigger pull on a loaded and cocked “ready to fire” weapon. The first setup, loading, cocking, and pulling the trigger is all on the shooter.  After that, the gun takes over in different ways.

With automatic type guns this process gets powered by harnessing excess energy created by the explosion that propels the just-fired bullet. The energy comes from the same action/reaction impulse that creates weapon recoil, or it comes via things like the pressure created from expanding gases from the gun powder detonation.  The gun basically just hijacks what would otherwise just be wasted energy and uses it to make firing again faster and more efficient. Much.

Here is a nice little animation that shows how it works:

The difference between full-automatic and semi-automatic modes is in how that process is handled - either once per trigger pull, or repeatedly until the trigger is released. That’s it.  

1. Trigger is pullled.

2. Firing mechanism is engaged to fire the loaded (“chambered”) bullet.

3. Eject the spent shell casing (the “leftover” of the cartridge).

4. Load another bullet into the chamber (from the magazine).

5. Cock the weapon to prepare to fire again.

5. Firing mechanism is disengaged or “blocked” until trigger is released.

The big thing here is that after the first shot, at the end of the cycle (#5), there is a break in the continuous flow and the shooter must physically release the trigger before the gun is able to start again at #1.

Non-gun people tend to think, and the gun lobby really wants you to think, that this is equal to slow.  It’s not, not by a long shot, and there are many good examples on Youtube, and in pop culture that demonstrate just how slow it ain’t.

Semi-automatic fire can be as slow and methodical as you like, but rapid semi-auto is often considered to be around 45-60 rpm, for an average shooter. But that is not actually a challenging speed -  because what is often cited there is actually an effective rate of fire.

The effective rate of a gun is different from it’s maximum, and it is less precise because it just means as fast as you can go while still being pretty sure you are actually hitting your target. So it includes aiming/re-aiming and maintaining target alignment, which means that things about the gun itself have a huge impact, but so do things about individual shooters. Effective firing rates can vary quite a bit more from person-to-person than from gun-to-gun.

Still, 45-60 rpm is actually just average, and not trying for burst-type shooting.

Really fast shooters can fire pretty effectively at multiple targets at a rate of 5-8 rounds per second - which is more like 300 rpm. That is also usually done as "double-taps" or 2 quick shots per target , and is far more than the 45-60 of a methodical sustained rapid-fire. And this is just regular shooting, there are tricks and modifications covered elsewhere that can ramp that up enormously.

Basically, with a semi-automatic, after the first trigger pull the gun fires and then cycles through the process outlined above and is then ready to fire again, but doesn’t. This happens extremely quickly, but there is a physical block put in place that forces the shooter to manually intervene before the next shot can be fired. In other words: the process is interrupted to create semi-automatic fire. When you don’t, you get fully-automatic.

This happens at incredible speed due to the high energy from the firing of the bullet. How rapidly it does this is called the “cyclic rate” and is measured in rounds per minute or “rpm”. For example, the M16/M4 rifles fire between 700-950 rpm. That will empty a standard high-capacity 30-round magazine in bit more than 2 seconds if the shooter doesn’t release the trigger. Definitely rapid.

This is typically much, much faster than the effective rate, which is discussed elsewhere in more detail but boils down to recoil effects.

Everybody knows what full-auto fire looks like:

It’s a rule that full-auto fire must be accompanied by screaming.

Full-auto isn't just one speed though - it's just one pull of the trigger and the gun keeps shooting, but how fast depends on different things, and varies a bit. It is always snappy, but can range pretty widely with the type of gun, but also just with the ammunition type. That's why the cyclic rate for the M16 is both estimated, and given as a range. The explosive characteristics of different types of ammunition create different cyclic rates, some faster, some slower.

Without mechanical intervention, the full-auto thing would more naturally be a runaway process; basically emptying whatever ammo the gun has with every pull of the trigger. This was and is an outcome of mechanical failures on automatic weapons, and a lot of design improvements over the years have been geared to making that less likely to happen.

So the basic design of automatic weapons creates a potential for a run-away shooting process and full-auto weapons include the ability to interrupt that process by releasing the trigger. Semi-automatics necessitate the interruption.

By including or changing a few parts they make it vastly easier to limit the rate of fire to one methodical shot at a time, or anywhere in between.

Here’s a video that shows the selective-fire function, and it also helps show the difference between full and semi-automatic fire clearly (you can watch it all, but the part of interest for this point is just the first 5-10 seconds). Note that the guy in the video is not necessarily firing as fast as he can in semi-automatic, that isn’t the point of this particular video, but we'll come back to that very soon anyway.

So, just to summarize and avoid some possible confusion: you may hear people refer to what a lot of the public thinks of as “machine guns” as: automatic weapons, fully-automatic weapons, or selective-fire weapons. And what people may think of as “machine gun fire” may be called full-auto fire, or selective fire (or even “cyclic fire”, but that rarely outside of military jargon). What you will never hear from the gun literate is “machine gun” unless they really mean a machine gun - which is something specific, and different from an assault rifle.

To whit, selective fire is the only functional distinction between poster-child guns in the feature image: the civilian AR-15 and the military M16. Colt very helpfully makes this point in one of their old ads:

Understanding how small the difference really is between civilian and military assault rifles goes a long way to explaining why they have so much overlap in killing power. It's literally that one function, and a few parts: