AR-15 vs. M4: What Are the Differences?

If you’re like most gun enthusiasts, you’ve at least heard of the AR-15. It’s an easy-to-use, modular semi-automatic rifle that’s versatile enough for sport shooting, hunting, and home defense. You might even have one or more of them in your gun safe.

But when you’re shopping around for a new rifle, you may have encountered an M4. This can lead to confusion since the two guns look almost identical. So, what’s the deal with the AR-15 vs. M4?

In short, the M4 is a select-fire military rifle, while the AR-15 is a semi-automatic civilian rifle. The M4 is also shorter, with a 14.5-inch barrel that’s not available on the civilian market. Let’s take a closer look at both firearms and see how they compare!

Ruger AR-556

AR-15 Information

The AR-15 was developed in the 1950s by the Armalite Corporation.

“AR” is short for “Armalite rifle,” and the gun is based on the earlier AR-10. The original model fired a 5.56x45mm NATO caliber and sported an air-cooled, gas-operated action that’s very reliable. In 1959, after failing to sell the AR-15 to the military, Armalite sold the design to Colt, which continues to make AR-15 rifles to this date.

Since then, the AR-15 has gone in two different directions. On the one hand, Colt made significant modifications to turn it into a new and improved military rifle, the M16.

On the other hand, the civilian AR-15 model remains popular for many applications. It’s reasonably lightweight as well as incredibly reliable. But most of all, because it’s been around for so long, it’s modular. You can order new and modified parts from many manufacturers, so it’s easy to modify for sport shooting, hunting, and other applications.

Colt M4 Carbine

M4 Information

The M4, properly called the M4 carbine, is an upgraded version of the M16 rifle. It was designed with a shorter, 14.5-inch barrel that’s easier to manage, while retaining the original 5.56x45mm NATO caliber. The standard version also comes from the factory with a telescoping stock, which is incredibly useful for soldiers in the field.

The M4 carbine is the standard-issue rifle for most parts of the US armed forces, including the US Army infantry and the United States Marine Corps. That said, it’s not available on the civilian market because of its select fire feature, which allows it to toggle between semi-automatic and three-round burst mode. The newer M4A1 variant is even capable of firing in fully automatic mode.

If you just give them a casual look, the AR-15 and the M4 look almost identical. It takes a trained eye to tell the difference. That said, when you look closer, there are some important distinctions to consider. Here’s what separates these two rifles.

Barrel Length

The first thing you’ll notice on a visual inspection is that the M4 carbine is shorter than the AR-15.

Carbines are shorter than standard rifles by design because they’re designed to be mobile. Think of a marine clearing buildings in Fallujah. They needed to move quickly and swing their gun around quickly in tight spaces. A carbine is perfect for the task, and the M4’s 14.5-inch barrel delivers.

The AR-15, on the other hand, is a true rifle. The standard version comes with a 16-inch barrel, and you can also order it with 18- or 20-inch barrels. The 16-inch barrel is what most people will want for home defense or sport shooting. On the other hand, the longer barrels offer improved accuracy at longer ranges. If your AR-15 is going to primarily be a hunting rifle, a 20-inch barrel could be worth it.

This represents a fundamental trade-off. A short barrel is easier to move around with, and can be better for home defense. A longer barrel, on the other hand, provides slightly higher muzzle velocity and better accuracy at range.

In this case, federal firearms laws make the decision easy. Anything under 16 inches is considered a short-barreled rifle and requires an FFL stamp. That’s why civilian M4 models come with a 16-inch barrel.

Select Fire Capabilities

The most important difference between the AR-15 and M4 is the way they fire. The AR-15 is a semi-automatic rifle. This means that the safety can toggle between “Safe” and single-shot mode, where one pull of the trigger fires one bullet.

The M4, on the other hand, has a select fire toggle. In addition to the single-shot mode, it can be set to a three-round burst mode that fires three bullets with one pull of the trigger. Some variants can even fire in fully automatic mode, where the gun will continue to fire until the trigger is released or the magazine runs out.

Under federal law, three-round burst and fully automatic firearms manufactured after 1986 are illegal for civilian ownership. Even guns manufactured prior to 1986 require a special tax stamp for ownership. In other words, if you’re not in the military, you’re not getting your hands on an M4.

Calibers

As a military firearm, the M4 carbine fires the standard 5.56x45mm NATO cartridge. As the name implies, this cartridge is used by NATO countries, where using the same cartridge simplifies supply lines and allows allied troops to work seamlessly together.

The AR-15, on the other hand, is available in many cartridges. While the original version was chambered in 5.56x45mm, .223 Remington variants are also common. AR-15 receivers and barrels are also available in .300 Win and similar common hunting cartridges, as well as smaller sporting cartridges. If you can think of a rifle caliber, chances are good you can find it in an AR-15.

Gas Tube, Feed Ramps, and Stock

Along with its shorter barrel, the M4 carbine has a correspondingly shorter gas tube. This presented an engineering challenge since the shorter tube causes cartridges to feed and expel at different speeds. To compensate, the M4 features a set of modified feed ramps.

It’s also worth noting that the M4 ships from the factory with a collapsible stock. Most AR-15s, on the other hand, ship with a fixed stock. This isn’t a big deal, though. There are literally hundreds of collapsible AR-15 stocks on the market for you to choose from.

AR-15 and M4 Similarities

Other than the factors we’ve discussed, the AR-15 and M4 carbine are remarkably similar. They utilize the same trigger assembly, charging handle, and bolt carrier group. And other than some minor differences, the upper and lower receivers are basically a carbon copy.

Most of those minor differences relate to the select fire feature, so you could almost say that an M4 is an AR-15 with a select fire feature. And while that’s not entirely true, the two guns have so much in common that it’s not far off the mark.

AK-47

The AK-47 is a Soviet-era rifle that was first developed in 1949 by Mikhail Kalashnikov. Like the AR-15 and M4, it’s air-cooled and gas-operated. And like the M4, it’s chambered in a larger cartridge, the 7.62x39mm.

This rifle is most famous for its reliability. You can drop it in mud or sand, pick it up, and shoot it. It’s easy to field strip and takes just a few minutes to clean. This makes it very attractive if you’re going on a long hunting trip and spending days in the field. Then again, it also weighs two pounds more than an AR-15, which is not insubstantial when you’re lugging it around all day.

Most AK-47 models are fully automatic, which makes them illegal to import for the civilian market. That said, there are many pre-1986 automatic AK-47s floating around that you can purchase with the appropriate tax stamp. And if you don’t need a fully automatic rifle, there are also semi-automatic civilian model AK-47s that you can order brand new.

If you’re a civilian, the choice is easy. Since the M4 is only available to the military, you’ll have to use an AR-15. That’s not necessarily a bad thing, though. Other than the ability to fire in fully automatic mode, the AR-15 is just as capable. And with a few tweaks, you can make it even better.

First off, you can upgrade your lower receiver and handguards to a mil-spec standard. These parts are readily available, and anyone can order and install them.

If you’re willing to jump through a few more hoops, you can customize your AR-15 with a shorter, 14.5-inch barrel. You’ll have to buy a $200 tax stamp and pass an NFA background check. But assuming everything checks out, you’ll have a short-barreled AR-style rifle, just like the M4.

There are even fully automatic AR-15s available that are legal for civilians because they were produced before 1986. They cost thousands of dollars, and you’ll have to pass the NFA background check and buy a tax stamp. But if you’re looking for the authentic M4 experience, this is as close as it gets.

As you can see, the AR-15 and M4 have a lot in common, and they look the same to a casual observer. They even fire the same caliber, assuming your AR-15 fires a 5.56×45mm NATO round.

Minor differences aside, there’s one overriding reason to choose an AR-15: they’re actually legal for civilians to own. Even if you buy a civilian-model “M4,” you’re getting a semi-auto rifle with a 16-inch barrel. In other words, a rebranded AR-15.

So why not stick with the original? Whether for sport shooting, home defense, or hunting, an AR-15 is as reliable as it gets.