Hand-Picked Daily GUN DEALS, and Exclusive Coupons Codes >>>

[What’s the Difference?]: AR-15 vs M4

You hear a lot about the M4 and especially the AR-15–both in the firearms community and in the news–but have you ever wondered what the differences are between them?

At first glance, these two rifles seem virtually identical. The truth is they are very similar, so there’s a lot of confusion about what differentiates them.

Simply put, the difference between the two is the M4 has either a full-auto or burst fire mode while the AR-15 does not. There are also minor differences such as barrel length and attachments, but these do not fundamentally affect the rifle.

(top) M4 Vs. (bottom) Ar-15

We’re going to talk about the histories and differences of the M4 and AR-15 so that you can differentiate between these two fantastic rifles–and correct your friends when they use the names wrong.

Let’s dig in!

Origins of the Rifles

To start, let’s talk a little bit about the history of these two rifles to get a sense of their purpose.

History of the AR-15

The AR-15 was first designed in 1956 by Armalite as a scaled-down version of the AR-10. The “AR” in AR-15 stands for Armalite Rifle, while “15” is a model number. It doesn’t actually stand for “assault rifle,” which is a popular misconception.

The AR-15 was designed in response to a request by the CONARC (U.S. Continental Army Command) for smaller .223 rifle to test that could replace, all in a single rifle, the Browning Automatic Rifle, M1 Garand, M1 Carbine, Thompson Submachine Gun, and M3 “Grease Gun.”

Imagine replacing all these circled weapons plus the Tommy Gun with just one rifle.
Imagine replacing all these circled weapons plus the Tommy Gun with just one rifle.

Unfortunately, the rifle was nixed in favor of the M14, despite the fact that the AR-15 performed far better in testing.

As a very small company, that setback–combined with limited funds and production capacity–led Armalite to sell the AR-15 (along with its predecessor, the AR-10) to Colt just a few years later in 1959.

Colt made some improvements on the design, then began mass producing the rifle and pitching it to military organizations.

The Colt AR-15 quickly began to see success in small markets. After seeing a demonstration of the rifle in 1960, General Curtis LeMay, at that time Vice Chief of Staff of the U.S. Air Force, ordered 8,500 for Air Force pilots to use as survival rifles.

Early AR-15s
Early AR-15s

LeMay would continue to champion the rifle in the US military, even attempting to order 80,000 units in 1961 after being promoted to Chief of Staff of the Air Force, but would continue to be shot down.

Until 1963, that is. The US Secretary of Defense, Robert McNamara, was almost a decade into the Vietnam War and had to face three facts:

  1. The M14 could not compete with the enemy’s AK-47
  2. The AR-15 consistently outperformed both the M14 and the AK-47 in testing
  3. M14 production simply couldn’t meet the demands of the US Military

McNamara approved the rifle and it was adopted as the M16 (or as the M16A1 or XM16E1 for the variant with forward assist).

The semi-automatic version of the Colt AR-15 for civilian and police use hit store shelves in 1964, where it quickly gained popularity. When Colt’s patent on the rifle expired in 1977, other manufacturers quickly began selling their own versions.

Modern AR-15 (Diamondback DB15SB M4 Basic AR-15)
Modern AR-15 (Diamondback DB15SB M4 Basic AR-15)

These days, just about every firearms manufacturer has their own AR-15 style rifle and there’s a huge market for AR-15 parts and accessories due to the rifle’s modular design.

History of the M4

The history of the M4 builds on that of the AR-15 and M16, so there’s not as much to say here.

After the M16 was adopted, the military quickly demanded a carbine version for close quarters combat, so Colt began producing the Colt Commando XM177 in 1966. 

Colt Commando
Colt Commando

The Commando did the job, but it had some issues with range and accuracy due to the shortened barrel, so in 1984, Colt began development of the XM4. Their goal was to combine the advantages of the Commando and of the new, improved M16 design, the M16A2, into the same carbine.

The XM4 underwent a decade of testing and modifications, as well as being renamed to the M4, before it was adopted by the US military in 1994.

M4 Carbine
M4 Carbine

Both the M4 and the M16 (now up to the M16A4 model) are still in service today, and the M16 is the longest continuously serving rifle in US military history.

So now that you’ve got a feel for where these rifles come from, let’s move on to the real reason you’re here: what makes the M4 and AR-15 similar and different?

Differences Between the AR-15 and M4

As we’ve seen, the AR-15 and M4 are in the same family, so it’s shouldn’t come as a surprise that there are very few differences between the two rifles.

You often hear the M4 described as the military version of the civilian AR-15 and vice versa, but that’s not really accurate. In reality, the M4 is the carbine version of the M16, the real military version of the AR-15.

Colt M16A2
Colt M16A2

If these rifles were a real family, then the AR-15 is more like an uncle, with the M16 being the AR-15’s brother and the father of the M4.

But what does that mean for the actual differences between the designs of the two guns?

For one, the M4 is a carbine-length rifle, so it has a smaller, 14.5-inch barrel, but the standard AR-15 barrel is 16 inches.

With so many different versions and such a large market for AR-15 upgrades, longer barrels can also be found, but US laws on short barreled rifles place a 16-inch minimum on barrel length on civilian weapons.

As a military weapon, the M4’s barrel can also be mounted with a grenade launcher thanks to a cut out in the barrel.

M4 Carbine with M203 Grenade Launcher
M4 Carbine with M203 Grenade Launcher

In proportion with the shorter barrel, the M4 has a shorter gas tube and to compensate for the shorter gas tube, the M4 also has modified feed ramps.

The M4 has a collapsible stock, while the standard AR-15 has a fixed stock–but collapsible stock configurations are widely available for the AR-15.

Finally, the primary, and best known, difference between the AR-15 and M4 lies in selective fire capabilities: the AR-15 is semi-auto while M4 can be fired in either three round burst or fully automatic firing options.


The similarities are basically everything else.

The bolt carrier group, charging handle, and trigger assembly are all the same. The internal parts of both the upper and lower receivers are virtually identical, and the few discrepancies are those required for the differences discussed above.

Depending on the configuration of the AR-15, even the handguards and rail systems can be the same.

M4 vs. AR-15: Which is Better?

Unless you join the military or are already serving (and if the latter, then thank you!), then you’ll have to settle for the AR-15 as the M4 is only available for use by military personnel.

Putting on that Uniform Is the Only Way You’ll get to shoot a real M4.
Putting on that uniform is the only way you’ll get to shoot a real M4.

Don’t sweat it too much, though, because the AR-15 is virtually the same gun without the fully automatic capabilities, and there are some options for making it even more similar.

The easiest and most accessible way is to buy a mil-spec lower and M4 handguards, which are both easy to find, and upgrade your AR-15 with them.

Under federal law you can also purchase or create an AR-15 with an M4 length barrel–assuming you fill out the correct form, pass the NFA background check, and purchase the $200 tax stamp. 

You can even have a fully automatic AR-15, but this is considerably more difficult and expensive. For a fully automatic weapon to be civilian legal, it has to have been produced before 1986. Fully automatic AR-15s that meet this requirement do exist, but they’re relatively rare and in very high demand. 

Full auto Sp01
Full auto SP-01, sold at Rock Island Auction for $34,500

Getting one can easily set you back $14,000. Though I guess, in context, the NFA background check and addition tax stamp that is also required don’t seem so bad.

Looking for other ways to upgrade your AR-15? Then check out our guide to AR-15 upgrades and accessories, and don’t forget to share your questions and thoughts with me in the comments.

The Best Gun Deals, Coupons and Finds

Subscribe to Pew Pew Tactical's sales and deals email.

10 Leave a Reply

  • jon arbuckle

    Forgot to mention that in the 90's Colt discontinued millling out the upper shelf behind the sear which prevents the installation of
    drop-in precision and binary triggers .

    June 27, 2021 6:26 am
  • Liliya B.

    Great article! Thank you.

    August 18, 2020 10:23 pm
  • travis v lopez

    no mention of eugene stoner?

    August 2, 2020 9:05 am
  • Anthony Musselman

    Awesome article! I shot the m-4 for years in the military and just now was like... well... what the hell even is the m-4?

    Thanks for the awesome explanation!

    July 26, 2020 1:52 pm
  • Gregg

    In 1963 the u.s. involvement in vietnam was limited to advisors and not a full military operation. This paragraph from the above article is in error.
    Until 1963, that is. The US Secretary of Defense, Robert McNamara, was almost a decade into the Vietnam War and had to face three facts:

    July 20, 2020 10:55 am
  • Steve


    September 15, 2019 7:35 pm
  • DeserKeylon

    Thank you for all the information. I am looking to build my first weapon and this gives a good background and details as to why this gun is so popular.
    Thanks again

    July 14, 2019 7:10 am
    • David, PPT Editor

      Glad we could help!

      July 14, 2019 7:14 am
  • POGO

    The standard M4's fire selector is SAFE, SEMI, AUTO. Auto is 3 round burst there is no full auto except on the M4A1 that is used by special operations.

    May 1, 2019 4:27 pm
    • djnaldooo

      That’s what I thought

      February 23, 2020 5:57 am