AR-15 vs. M16: The Real Differences (And Similarities)

You may be familiar with the basic history of the AR-15: Eugene Stoner developed the .308-chambered AR-10 in the late 50s, shrunk the rifle down, chambered it in .223 Remington, and called it the AR-15. In 1963 (after ArmaLite sold the rights to the AR-10 and AR-15 platforms to Colt) the military adopted the AR-15 as a replacement to the M14, which was then suffering production issues and failing to meet the military's demand. The military made some basic design changes to the AR-15 and designated it the "Rifle, Caliber 5.56mm, M16."

Today, the M16 -- and its smaller variant, the M4 -- remain the standard-issue rifles for a bulk of the U.S armed forces. The AR-15, meanwhile, is now arguably the most popular firearm bought and sold on the civilian side of the American firearm market today.

So, what are the differences? And what remains the same between these two platforms?

AR-15 vs. M16 & M4: Key Differences

Although they may look identical -- and although you can even build a perfect "clone" rifle of an M16 or M4 with an AR-15 -- these two platforms are, in fact, different. Key differences include variations between the components installed on both rifle platforms, and some functional differences.

Functional Differences

A U.S Army soldier firing an M16. Note the "BURST" position of the safety lever, and third pin for the auto sear, located atop the receiver. 

Photo credit M. Rauchenberger

Semiauto vs. Full-Auto Fire

First and foremost, the AR-15 is not an automatic weapon. It can't even be made to function as an automatic weapon without modifying the lower receiver to accept certain components.

The M16 (and M4), meanwhile, are automatic weapons. They have "select-fire" capability, which means they can be made to fire semiautomatic or fully automatic. Most M16 and M4 variants fire in three-round bursts when automatic fire is selected, though some are manufactured to provide simple full-auto function.

That means for as long as the trigger is depressed by the shooter, the rifle will fire on its own. The M16 and M4 offer fire rates of 700 to 950 rounds per minute.

Safety Selector Lever

The M16's select-fire capability is handled by a three-position safety lever. When turned forward and horizontal, the weapon is safe. When flipped 90 degrees vertical, it is placed into semiautomatic fire. When flipped an additional 90 degrees toward the rear, it is placed into three-round burst or full-auto fire.

The AR-15's safety lever only offer the two former positions: Safe, and semiautomatic.

Lower Receiver Components

The AR-15 fire control group, which provides semiautomatic fire (left) and the M16/M4 fire control group, which offers full-auto fire (right). Note the notch in the full-auto hammer, and the auto sear (yellow) located atop the disconnector.

Photo credit Andy Sou

The M16 (and M4) provide three-round burst and full-auto fire thanks to key differences in the components found in their lower receivers, and the lower receivers themselves, none of which are found in the AR-15. Those include the:

• Hammer
• Trigger
• Disconnector
• Auto Sear
• Third Pin
• Third Pin Hole
• Full-Auto-Capable Stripped Lower Receiver

("Can I convert an AR-15 into a full-auto weapon?")

No, not without extensive modification. The AR-15's stripped lower receiver cannot accept an M16 auto sear, which is necessary to provide full-auto function. It also lacks the notched hammer and modified disconnector and trigger, as well as the third pin hole for the full-auto fire control components, which are all required to install an auto sear.

Converting any AR-15 to provide full-auto function, like an M16 or M4, requires extensive modification to the cavity and walls of the receiver. This is illegal to do as an end user or civilian owner, and should never be attempted by anyone other than a Federal Firearms Licensee who possesses the appropriate "Special Occupational Taxpayer" (SOT) designation required to legally build and modify machineguns.

Bolt Carrier Groups

The M16 and M4 require a specific bolt carrier group designed to interface with the full-auto fire control components in the lower receiver. This, naturally, is referred to as an "M16," "full auto," or simply "F/A" bolt carrier group. These groups have more mass at the rear portion of the carrier (pictured above, left) while AR-15 bolt carrier groups have less mass (above, right) and cannot be made to work with M16 fire control components.

Differences in Rifle Configurations

Beyond the differences in fire control components and BCGs, the AR-15, M16, and M4 share nearly all the same components; we'll touch more on this later. But each rifle differs in how it's configured.

AR-15 vs. M16 & M4 Barrels

The standard AR-15 is configured with a 16" barrel, which is the minimum legal length for any rifle barrel. To install a shorter barrel on a rifle, it must first be registered as a short-barreled rifle ("SBR").

The M16 is configured with a 20" barrel, which maximizes the muzzle velocity and accuracy of the 5.56 NATO cartridge. The M4 has the shortest barrel, with a standard configuration measuring 14.5". Some variants of the M4 have even shorter barrels, like the U.S. SOCOM MK18, which has a barrel measuring just 10.3" long.

AR-15 vs. M16/M4 Cartridges

The chambers found in these barrels differs, too. The AR-15 can be chambered in 5.56 NATO, .223 Remington, or .223 Wylde, a hybrid chamber that allows both cartridges to be chambered safely, with better extraction and accuracy.

The M16 and M4 are exclusively chambered in 5.56 NATO, the standard-issue ammunition for the U.S. military's rifles across all branches.

To answer the question: .223 Remington can be chambered in a 5.56 NATO barrel, but 5.56 NATO should not be chambered in a .223 Remington barrel. The .223 chamber has a shorter lede -- the area where the bullet rests just behind the rifling -- which can result in over-pressurization of some 5.56 loads.

We cover the differences between 5.56 NATO and .223 Remington in detail, here.

AR-15 vs. M16 & M4: What's The Same?

We've covered all the differences in parts when comparing the AR-15, M16, and M4 -- which, again, includes the fire control components and stripped lower receiver -- but, surprisingly, virtually all other components found on all three rifles are, functionally, identical. They include the:

• Stripped Upper Receiver
• Charging Handle
• Barrel Nut
• Gas Block
• Gas Tube
• Muzzle Device
• Buffer Tube
• Recoil Spring
• Weighted Buffer
• Pistol Grip

It's also perfectly legal to own an M16 bolt carrier group. In fact, the M16 BCG is now the most popular bolt carrier group on the civilian market and it's almost exclusively what every modern parts maker, rifle manufacturer, and firearm retailer offers today. It's what we provide with all our barreled uppers and AR-15 kits.

AR-15 vs. M16: Myths Debunked

With how similar the AR-15 and M16 are to one another, it's not surprising there are plenty of misconceptions and outright myths surrounding both rifles. Let's clear some of them up:

No, The AR-15 is Not An "Assault Rifle"

An assault rifle is a fully automatic weapon issued to military personnel. Given their three-round burst and full-auto capabilities, the M16 and the M4 both qualify as assault rifles. The AR-15, which can only ever be configured as a semiautomatic weapon, is not an assault rifle.

No, M16 and M4 Clones Are Not Full-Auto Capable

While it's true that M16 and M4 clone builders can assemble an AR-15 almost exclusively from military-issued components -- including purchasing stripped lower receivers with military markings and auto sear cutouts -- it is not true, ever, that these "clone" rifles can be made to provide full-auto function. These receivers lack a third pin hole and, as we mentioned earlier, modifying any AR-15 to provide full-auto fire without an FFL SOT is illegal.

It is also a legal gray area to even own an M16 auto sear while possessing an AR-15, especially if you possess a lower receiver with the cutout for installing a sear. The federal government likes to prosecute individuals in these cases by citing something called "constructive intent," and claiming you intended to modify your rifle make it a machinegun.

Yes, It Is Legal (But Very Expensive) to Own an M16

Even if you're a civilian without an FFL SOT license, you can legally purchase and own an M16 or M4 (if machineguns aren't banned in your state). These are referred to as "transferable machineguns," which are fully automatic weapons manufactured prior to the May 19, 1986 ban on machineguns. Any machinegun made prior to this date can be bought and sold on the civilian market, just like any other NFA weapon, including SBRs and suppressor. But because of their age and rarity, most of these firearms cost tens of thousands of dollars.

At the time of this publication, a transferable, fully automatic M16 lower receiver assembly costs around $30,000. Complete M16 rifles in good condition fetch more than $40,000; like-new examples demand $60,000 or more.

Because the M4 was only accepted for military manufacture and use after 1986, pre-ban, full-auto examples cannot be found on the civilian market. The first M4 prototypes were manufactured in 1985, one year after the federal ban took effect; the rifle was officially adopted by the U.S. military in 1994.

Yes, It Is Legal to Own a Short-Barreled AR-15

You can convert an AR-15 pistol into a short-barreled rifle, or you can convert a 16"-barreled rifle into an SBR. Or, you can simply build an SBR from scratch -- you simply need to register it with the ATF, per the National Firearms Act of 1934, before assembling it. We provide an in-depth guide on how to do exactly that.