What’s the Difference Between an AR-15 and an M4 rifle?
Both the AR-15 and the M4 are based on the Armalite Rifle design from Eugene Stoner in the 1950s. These rifles have, in their respective roles, served the United States well since the Vietnam War.
In service arenas, they are both select-fire, automatic combat rifles capable of fully automatic fire, but they differ in terms of historical context, features, and handling characteristics. These high-profile firearms have been dissected and discussed at length by both firearms enthusiasts and within the public arena – and here we’ll detail the histories, similarities, and differences between these two rifles and offer you some examples that can be purchased and used by civilians today.
Origins of the Rifles
The original Armalite Rifle 15 (Armalite Rifle being the “AR” in AR-15, not “Assault Rifle” as is often assumed) was developed by Eugene Stoner in the 1950s as a replacement to the venerated M1 rifle from the Second World War.
The AR-15 paired full-auto capabilities with a huge variety of calibers and barrel lengths, making it an early favorite of special forces in Vietnam and effective replacement for no fewer than 5 different firearms (everything from a Browning BAR to the M3 Grease Gun) and was adopted by the United States Armed Forces as the M16 rifle.
ArmaLite eventually sold its rights to the AR-15 to Colt in 1959 owing to financial issues and an inability to meet production demands with the incredibly popular rifle.
The smaller 5.56/.223 round was easier to control as compared to the M14’s 7.62×51 cartridge, although the AR-15’s reliability was an issue for many, especially in the unforgiving jungle terrain. During the Vietnam conflict, in particular, the combat inadequacies of the M16 rifle became clear, which resulted in both design revisions to the rifle as well as changes to ammunition and training adjustments.
Originally issued without cleaning equipment (soldiers were told the M-16 did not require maintenance) fouling from ammunition resulting in huge issues with reliability.
The problem was bad enough that soldiers routinely requested cleaning equipment from home to maintain their rifles.
In addition, the AR-15’s aperture rear sight was designed for quick target acquisition in variable light conditions of jungle fighting, but often lead to accuracy issues as users would confuse the sight wings for the post – often with unfortunate consequences. Its handy carry handle made it a lot easier to hump through the jungle than the M14 – plus it weighed substantially less than its predecessor.
By the end of the war, the AR had taken on several distinct varieties, including everything from a 9mm variant to a heavy barreled version meant for use as a light machine gun.
There were also adjustments to ammunition weights and barrel twist as more feedback worked its way back into the design. By the end of the conflict it was clear that the AR was here to stay, and it would soon mature into the rugged combat weapon we know and love today.
History of the M4
At a first glance, the M4 looks like a shortened M16, and that’s exactly what it is (technically an A2 configuration with a shorter handguard, shortened barrel, and relocated gas block).
But the M4 certainly wasn’t the first attempt to configure the rifle for a specific application – it was the result of half a century of learning.
The first short AR-15s were the Vietnam era CAR-15 and its cousin, the XM177. Both had a similar problem: as you shorten the barrel of a 5.56 caliber AR you lose projectile velocity, kinetic energy, non-suppressed sound levels, and increase flash intensity.
These are, of course, huge liabilities to introduce when operating in either close quarters or clandestine operations.
To update the CAR concept, the US military added some barrel length, eventually landing on a 14.5” inch barrel, which stabilized standard ammunition and enabled long range target engagements while still shortening the rifle to a carbine profile rather than the full-length 20” barreled rifle.
This new M4 also introduced, shortly after its adoption in the 1990s, rail systems that allowed the attachment of optics, accessories, and fun toys like under barrel M203 grenade launchers, making it the choice infantry weapon for militaries, law enforcement, & special operations units around the world.
The M4 is, in effect, the grandson of the M14. A grandson that spent some time at the gym slimming down and nailed a svelte fighting weight, but keeping the best bits like largely interchangeable components.
Spend enough time perusing /r/guns and you’ll encounter franken-guns that will make any arms historian seriously scratch their heads.
Working from muzzle to butt, there are some differences between the AR-15 to the M4.
First, the original AR-15 (Assuming M16A1) uses the classic three-pronged flash hider while an M4 has a newer style bird-cage hider.
Second, the barrel: the M16A1 had a longer, slimmer profiled barrel whereas the M4 has a 14.5” barrel with an under-barrel grenade launcher cut-in ala Dutch. Moving to the barrel, the AR-15 has longer handguards and a rifle-length gas tube – and the M4 uses shorter handguards (owing to the shorter barrel), a carbine gas system, and the availability of rail mounts.
The receivers look nearly the same, but the M4 has improved feed ramps. Modern M4s will have a flat-top receiver with a 1913 optics rail, but some very early models can be found with old M16A2 uppers.
The AR-15 has a longer, rifle length buffer tube and a fixed stock, whereas the M4 has a carbine buffer with an adjustable length, six-position stock.
In effect, the M4 is an AR that has been cut down to a carbine without sacrificing ballistics and has been modernized for rails, attachments, and stock adjustment.
Again, working from muzzle to buttstock.
Most M4s have the same birdcage muzzle device as the M16A2. Both fire the same caliber, 5.56mm, but have varying twist rates, and the internal components in the receivers are the same, as in the charging handle.
The buffer itself is the same, and the springs can be, though you should run appropriate length springs in an M4.
Which is better?
Selecting between the AR-15 and the M4 is a little like picking between your two favorite muscle cars. The AR-15 is more like the 1970 Dodge Charger that’s ready to jump a ditch and get away from the county sheriff, whereas the M4 is the 2020 Charger Scat Pack that will do 0-60 in under five seconds on street tires.
A faithful AR-15 classic
Practically unchanged after 50 years, a 20” AR is still a capable battle rifle and we’d be happy to carry Brownell’s awesome reproduction into most situations. The Brownell BRN-10® is available in both an AR-10 .308 configuration as well as the 5.65 model, which are faithful recreations of the original, with some key updates.
Brownells ticked all the right boxes with this light, comfortable rifle that offers fully machined 7075 T6 aluminum billet receivers (definitely not an option on Stoner’s 1950’s rifle). Toss in their reproduction sight, 20 round box magazines, and some aviators and you’ll feel like the coolest guy at the range.
A viable civilian M4
To approximate an M4, look no further than Palmetto State Armory PA-15 Carbine that has the correct barrel and sight profile but offers an aftermarket rail rather than mil-spec.
You couldn’t ask for much more of a home defense rifle – pair the modern, 30 round magazine with either irons or a quality red dot and you’re cooking with gas.
The AR-15 and M4 are both evolutions of the original AR design and both have their place in any collection.
That said, the M4 offers a newer evolution of the platform and gives the user the most modern interpretation – but both are outstanding rifles.
- Wikipedia, Armalite AR-15
- Historical Firearms, Portuguese AR-10
- Springfield Armory, Standard Colt AR15 M01
- Springfield Armory, U.S. Assault Rifle SMG GAU – 5/A 5.56MM
- Wikipedia, M4 Carbine
- School of Advanced Military Studies
- Small Arms Defense Journal, Barrel Length Studies in 5.56 NATO Weapons
- NRA Museum, Colt AR-15
- NRA Museum, Gunbook Glossary
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