What are the differences between AR15 and M4? 

What are the differences between AR15 and M4?

 

22 Answers

Keith Shannon, Avid paper-puncher, Texas LTC

Answered Feb 13

The M4 is a very specific weapons specification of the U.S. Armed Forces. The AR-15 is, most accurately, a design family.

• All M4 rifles are select-fire, meaning that they have a burst-fire or full-auto setting (usually burst even though it’s labelled “auto”). AR-15s can have an automatic-fire setting, but since 1986 there’s been virtually no market for full-auto ARs outside the mil-spec, as any new ones are illegal to possess outside law enforcement SWAT teams, specialized security contractors and a few licensed manufacturers and dealers who provide such weapons to such agencies. The overwhelming supermajority of the estimated 25 million ARs in civilian hands (we’re talking 3 or 4 “9”s as in >99%) have been semi-automatic only.

(M4 receiver: notice the three-position fire selector above the grip)

(Fire control group of a DPMS-made civilian AR-15; notice the conspicuous lack of an “Auto” position)

• M4 rifles are, by definition, short-barreled rifles under the U.S. National Firearms Act, with a spec’ed barrel length of 14.5″ (and as short as 11″ for some produced rifles). So, even a semi-auto variant of one is still a “Title II” weapon and requires an NFA tax stamp costing $200 and currently requiring about a 6 months wait time for the registration paperwork to get through the ATF. Most AR-15s have 16″ barrels, the minimum legal barrel length for a “rifle” under Title I of the NFA (not requiring registration and taxation), with a few bumping that to 16.5″ to be absolutely sure it’s legal even with discrepancies in measuring equipment and methods. A few are the M16’s spec’ed length of 20″.

(M4A1 on top, M16A4 below)

• M4 rifles, while they have “flattop” receivers with a Picatinny rail allowing mounting of common optic devices (red dots, scopes, BUIS), were originally spec’ed with a combination of a half-length “quadrail” handguard and an M16A2-style front sight integrated into the gas block assembly attached to the barrel. This means the front sight is fixed in place even if another sight is being used. For the M4, designed for close-range engagements and so a non-magnifying red-dot sight is standard-issue, this isn’t as much of a problem as the red dot is set up to “co-witness” with the iron sights (looking through the sights and red dot, the red dot is right on top of the front sight post and so they have the same aimpoint).
  
  AR-15s, for their part, have much wider proliferation of handguards and sighting options. The cheapest factory rifles tend to have an M16A2-style “cone” handguard with fixed front sight (and commonly a fixed rear as well using the A2-style carry handle upper receiver), and there are a few quadrails still sold in the current market, but most of the civilian AR market in the butter zone of $800-$1200 MSRPs has gravitated toward one of two aluminum handguard systems: “KeyMod” and “M-LOK”. Both typically have a Picatinny rail along the top of the handguard for mounting sights, but their other surfaces are relatively smooth, if skeletal, giving the rifle a natural horizontal grip like a traditional rifle, and unlike the quadrail which is very uncomfortable to hold while firing and requires special grip panels to allow its use as a horizontal grip. The newer systems are also not dependent on fixed front sights, allowing other options including flip-ups that can be collapsed down out of the way of a scope or magnifying sight, or foregoing iron sights altogether in favor of glass. The handguard systems are not mutually compatible with regard to other accessories, but most of the common accessories like vertical or angled foregrips, light mounts, strap anchor points, bipods etc are readily available for either system, and both systems also allow attaching sections of Picatinny rail in the exact lengths and positions needed for mounting more universal Picatinny-compatible accessories.

(USGI M4 with quadrail, fixed front sig...

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Aaron Brown

Answered Aug 20, 2018

M4 and M16 are just military designations the Canadians call them C7/8’s they are both the AR-15 platform rifles.

Select/Automatic fire is what separates a typical civilian AR-15 and a Police or Military AR-15/M4/M16

A typical AR-15 is semi auto only and cannot take critical full auto fire control parts without machining space for them.

The lower receiver on the Semi Auto AR-15 will not accept the critical component of the M4/M16 that allows the select/automatic fire which is the auto sear.

This grabs onto the hammer and holds it back until the bolt has locked and gone into battery. On the M4/M16 the bottom of the bolt carrier trips the auto sear releasing the hammer and fires the gun.

AR-15 on the left M4 on the right.

The trigger/hammer/safety/selector/disconnect parts are interchangeable for the most part some manufacturers have put blocks that prevent full auto safety/selectors to fit into their lowers and all manufacturers have left aluminum in the semi auto AR-15 lower to prevent installation of the auto sear.

Without the auto sear putting full auto parts into your semi auto AR-15 wont do anything.

There are also AR-15 semi auto bolt carriers that are machined to not interact with automatic fire control parts so they will only be capable of semi auto even in guns with all the other parts.

On the M4/M16 bolt carrier that bottom shelf that is milled down/off the AR-15 carrier is what trips the auto sear.

There are also very important differences between the trigger groups I think the only parts they have in common are springs and pins.

They are interchangeable to a point but wont retain select fire capability without all of the correct parts and milling.

The M16 hammer has an extra hook on the top right just below the H, that is what the auto sear grabs onto when you are on full auto holding the trigger down, it only gets released when the bolt goes fully into battery and locks.

The parts are similar but distinct and they have been designed to make conversion to full auto very difficult.

There are some devices like the lightning link that simulate what an auto sear does in semi auto AR-15’s with semi auto triggers.

But they are NFA items and insanely expensive for how simple they are since they were banned from new production along with machine guns in 1986. They are also not as good/reliable/safe as a properly made trigger mechanism.

I hope that helps you understand the mechanical differences between the typical AR-15 and the Military/Police rifles.

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Jonathan Bennett, worked at U.S. Army

Answered Oct 4, 2017

An AR-15, or Armalite Rifle model 15 is a rifle meant for civilian use. It weighs about 6.55 lbs and is only semi-automatic.

The M16 is a scaled down version of the AR 10. The AR 10 fired 7.62x 51 mm rounds, while the M16 fired the 5.56x45 mm round. Some early variants were capable of fully automatic fire, but eventually this was found to be largely unnecessary in most situations. Later variants can select for semi-automatic and burst fire.

The M4 carbine is a shorter and lighter variant of the M16A2 assault rifle. Most of these rifles in military use have fire selection for safe, semi-automatic, and three round burst fire. Three round burst is rarely a good idea. The M4A1 is the only fully automatic M4 model, replacing the burst selection. It typically weighs 6.5 lbs before attachements.

Answered Oct 4, 2017

Ill break it down pretty simple.

the only real differences, are as follows.

The fire control mechanism and barrel length. the M4 is a 14.5 inch select fire weapon (currently illegal to own since it was introduced passed March 18th of ’86 to be registered as NFA), meaning only LEO and Military.

The AR 15, is a semi auto ONLY, 16.25inch to 20+ inch barrel rifle. It has all the other components as the M4 does, Mil-Spec receivers, triggers, barrels, gas systems, magazines, Bolt Carrier Groups. BUT it (AR15) cannot shoot full auto like its M4 cousin.

Heres some info, if your interested on some “rapid” fire stuff that can make an AR15 simulate M4 like shooting.

AR-15s have 2 new and to me, quite interesting upgrades. One is a bump-fire stock (ie, bumpfire, Bumpski) which allows you to simulate fullauto fire, while still being ATF approved. When shot, the rifle recoils into the stock, moving the trigger away from your finger, and upon return, you hit the trigger again, thus sending another round down range.

the 2nd one, which is pretty new to the market is the Binary trigger. You pull the trigger back, bang, release, bang. Simple as that.

That is 2 of the several other ways. It really doesn’t pertain to the initial question, but it kinda does at the same time. AR-15’s DO NOT HAVE FULL AUTO CAPABILITIES, M4’s DO HAVE IT.

All things considered they are indistinguishable from a distance. (Within reason)

The lower receiver is where everything changes. The trigger group, recesses cut into it internally to accommodate a full auto/burst mechanisms Some manufacturers have modified the bolt as well. The selector would tell you but again you have to be right up on it to tell the difference.

In the past military style weapons were produced where everything was identical to keep production costs low but rather than installing full auto assemblies they were installed with semi-automatic assemblies.

After market parts filtered into the community and were technically lawful for sale, but unlawful when installed and were meant to turn semi-automatic firearms into full automatic firearms mechanically.

NOTE: This is the entire argument surrounding bump stocks. With a bump stock, mechanically the AR-15 is still a semi-automatic rifle therefore the “conditions” under which it is considered illegal is sketchy and could in the future change the common thoughts on what can be called illegal and what will remain legal. There are lots of those who own firearms who agree bump stocks are a way to skirt full auto restrictions. The problem is unintended consequences could come back to bite firearm owners in the ass.

Once upon a time, there was a kit made for Volkswagen that would allow you to make your beetle look like a Porsche. JC Whitney used to have them in their catalog. Whatever else it was, it was still not a Porsche. The AR-15 can come with a flash hider, bayonet lug, have the barrel that fits the military’s 40mm grenade launcher (which no normal person would ever get) The point is there are plenty of items “made for the battlefield” that either were adopted from civilian industries or civilian industries that adopted military hardware for. The firearm after it was invented almost certainly became a defensive weapon for the military. Not because of how effective it was but because of the noise it made. (That’s my theory)

In any event, there are dozens of rifles that the manufacturers “hoped” would be adopted by the military. (The mini-14 is one of them) They all have common components so the argument that the AR-15 should be banned or restricted comes down to how it looks and not much more…..