The M4 Carbine and AR-15 rifle are two of the most influential firearms in modern history, and while they share quite a lot of history and many design features, there are also some crucial differences that set them apart. So if you’ve ever wondered what the actual difference is between these two platforms, you’re in luck – we’re going to break down the history, design philosophy, and features of the M4 and AR-15 to see just how different they really are.
A Brief History
To fully understand the design philosophy behind the AR-15 platform and the M4 carbine that would follow it, we need to start all the way back in 1954 with the development of the AR-10 rifle by the ArmaLite Corporation and their chief design engineer, Eugene Stoner.
In the aftermath of World War II, the United States military made the decision to replace the venerable M1 Garand with a new rifle that would be lighter in weight, magazine-fed, and capable of fully automatic fire. This search for a new service rifle coincided with the establishment of NATO, and one of that organization’s early efforts was to develop a standardized rifle for use by its member states.
Firearms manufacturers soon began receiving invitations to submit designs for the military’s new rifle trials, and entries included Fabrique Nationale’s prototype FAL and the Springfield Armory T44, which would eventually become the M14. But ArmaLite, which was at that time only a small and virtually unknown subdivision of the Fairchild Engine and Airplane Corporation, also decided to throw its hat in the ring.
The AR-10 design that ArmaLite submitted immediately stood out from the competition – its innovative adjustable gas system and the use of lightweight materials such as aluminum and fiberglass were a major departure from the typical wooden stocks and piston systems of its predecessors, and the initial response to the AR-10 was very favorable. Unfortunately, ArmaLite president George Sullivan’s decision to use an untested aluminum/steel composite barrel (something that Eugene Stoner strongly objected to) proved to be its undoing, as the barrel burst after firing less than 6,000 rounds during torture testing. While ArmaLite quickly replaced the barrel with an all-steel version, the damage was already done, and the AR-10 was passed over in favor of Springfield’s M14 prototype.
While the AR-10 would never enter service in the United States, it would soon serve as the blueprint for what would become the longest-serving standard issue rifle in U.S. military history, as well as the most popular civilian rifle in America.
Though the M14 was officially adopted into service in 1957, the first combat reports coming out of Vietnam from US military advisors and the 1st Special Forces Group that same year resulted in the US Continental Army Command putting out a request for a new rifle that was smaller, lighter, but still capable of penetrating a steel helmet at 500 yards.
ArmaLite quickly rose to the challenge, producing a scaled-down version of their AR-10 chambered in .223 Remington – and thus the AR-15 was born. While testing resulted in praise for the much lighter, more controllable, and easier to produce AR-15, Army command declared that all service rifles and machine guns should share ammunition, and the AR-15 was once again sidelined in favor of the M14.
Frustrated by the inability to secure domestic buyers for the platform, and strained by financial problems and lack of production capacity, ArmaLite sold the design and trademark for both the AR-10 and AR-15 to Colt’s Manufacturing Company in 1959. Eugene Stoner left ArmaLite shortly thereafter in 1961, and George Sullivan bought the company from Fairchild the following year.
Extensive military testing of the AR-15 would continue through 1963, and despite passionate support from Air Force Chief of Staff Curtis LeMay, the DARPA agency, and U.S. Special Forces, the rifle continued to be snubbed, until Secretary of the Army Cyrus Vance eventually ordered an investigation into why the AR-15 had been rejected by the Army. The investigation revealed that testing had been rigged by the Army Materiel Command, who preferred the M14 and deliberately chose tests that would favor it, and used match-grade M14s against out-of-the-box AR-15s.
This led to Secretary of Defense Robert McNamara halting producing of the M14 in 1963 and beginning procurement of the new rifle, which underwent several rounds of modification (including some inter-branch back and forth over the inclusion of a forward assist) before finally being standardized as the M16A1 in 1967 and formally replacing the M14 as the standard service rifle in 1969.
Development of the M4 Carbine
Even before the M16 rifle had been officially adopted into service, Colt had already begun to experiment with carbine variants for close quarters combat and for use by vehicle crews. These included the CAR-15 family of weapons (originally proposed as a modular Military Weapons System meant to showcase how versatile the rifle platform could be) such as the CAR-15 Carbine, CAR-15 Model 607 Submachine Gun, and the CAR-15 Survival Rifle. Colt also produced the Colt Commando, which was not originally part of the CAR-15 Military Weapons System but ended up being purchased in small numbers by the military for use with US Army Special Forces and the multi-service MACV-SOG operating in Vietnam.
These carbines, designated the XM177 series by the Army, were lighter and more maneuverable than a full-sized M16, but were plagued by problems with range, accuracy, muzzle flash, and fouling caused by the much shorter barrels. Colt asked for funding to conduct a study that would improve upon these issues, but with American ground involvement in Vietnam winding down, the military declined Colt’s request.
The next leap in development wouldn’t occur until 1982, when the government asked Colt to develop a carbine version of the newly upgraded M16A2 rifle. With US Army assistance, Colt designed the XM177E2, which was re-designated the XM4 Carbine to indicate the Army’s intention to replace the M3 Carbine (which was itself an evolution of the older M1 Carbine). After additional testing led to improved furniture, a ballistically superior 1:7 twist rate, and a slightly lengthened barrel, development of the new carbine was formally approved in 1984.
Production of the XM4 Carbine was originally a joint project between the Army and Marine Corps, but the Army withdrew their funding in 1986. The first production run of the new carbines was completed the next year despite this setback, and the Marines officially adopted it as the M4 Carbine.
Though the Army eventually did give Colt initial contracts for M4 production in 1993, it wasn’t until both the First Gulf War and the highly publicized Battle of Mogadishu that the Army fully grasped the need for a lighter and more compact rifle better suited to the urban environments and short-range engagements that had come to define modern combat.
The M4 first saw use by American troops as part of the NATO-led peacekeeping force assigned to Kosovo in 1999, but it wasn’t until Operation Enduring Freedom and Operation Iraqi Freedom that the M4 truly stepped into the spotlight, largely replacing the M16A2 among forward-deployed troops in both conflicts.
In 2009, the Army took over ownership of the M4 design, allowing other manufacturers to compete with Colt in terms of design and production. That same year, the Army proposed changes to the platform that included an electronic round counter, a heavier barrel, and a transition to a gas piston system, but this proposal was abandoned after widespread criticism.
Instead, the Army set about replacing the original M4 Carbine with the M4A1 Carbine, which had been in use by Special Operations units since 1994. The M4A1 features a fully-automatic trigger control group, a removable carry handle, and a heavier, more durable barrel to compensate for the added wear and tear of fully automatic fire, and through a combination of M4A1 production contracts and the use of conversion kits, the Army has effectively phased out the original M4.
That leads us to today – and now you might be wondering what exactly the differences are between the M4 Carbine and the AR-15 that has found a home in virtually every civilian shooter’s gun safe.
Directly comparing the M4 and AR-15 can be tricky for a couple of different reasons. The biggest issue is that we have to decide which M4 and AR-15 we’re looking at – the original M4 design with its two-position stock, fixed carry handle, and 3-round burst capability is very different from an M4A1 SOPMOD Block II with a 6-position stock, EOTech optic, and fully-automatic trigger group, and “AR-15” is vague enough to encompass anything from an AR pistol to a long-barreled competition shooting or varmint hunting setup.
If we really want to nitpick, there’s also the fact that Colt still owns the AR-15 trademark despite their patents on the design expiring in 1977, which means that all other manufacturers are technically selling AR-15 style rifles rather than an “official” AR-15.
Aside from the obvious lack of 3-round burst or automatic trigger groups on civilian rifles, the differences between a standard M4 and a typical off-the-shelf AR-15 are, as you’d probably expect, pretty minor. The M4 features a slightly shorter 14.5-inch barrel with a cut-out for mounting an underbarrel grenade launcher or shotgun (both of which can also be accomplished with an AR-15 provided you buy a government profile barrel and don’t mind spending the time and money to file the necessary tax stamps to own a short-barreled rifle), a slightly shorter gas tube, a slightly lighter buffer spring, and modified feed ramps. AR-15s also have some minor changes to the bolt carrier, hammer, disconnector, and safety selector in order to prevent the rifle from engaging with an automatic sear or using other illegal drop-in automatic conversion parts.
Aside from the select-fire capability, it is actually entirely possible to build an AR-15 that is all but a carbon copy of the M4, which is why it might be more useful to think of the AR-15 as a broader design family and the M4 and M4A1 as a specific platform with a very particular goal in mind. While an AR-15 is an endlessly modular platform that can perform virtually any task with the proper components, the M4 Carbine was designed only with modern warfare in mind, and the improved M4A1, along with the SOPMOD accessory kits have only made it even more effective in that role.
As we’ve seen, both the M4 and the AR-15 share the same fundamental design history and feature largely interchangeable components and functionality, with the major exception of the ability to fire in 3-round burst or full auto. While the M4 / M4A1 has been honed into an exceptionally effective combat rifle over their decades of service, the AR-15 has likewise only become more versatile and modular over time, providing the civilian shooter with almost limitless potential. Whether you want to do some inexpensive plinking at the range with a .22 WMR upper or hunt big game with .450 Bushmaster or .50 Beowulf, an AR-15 rifle or pistol is a fantastic choice.
While you're here leave a comment and check out our other articles like Rimfire vs Centerfire and our guide to .277 Sig Fury, the Army's newest adopted caliber.We sell the best value AR-15 and AR-10 rifles, uppers, and barrels here at Bear Creek Arsenal; shop your next build here!]