What is the difference between an AR15 and an M4?
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AR-15 is the Armalite designation of the Eugene Stoner design. The US Armed Forces adopted the Stoner design as the M16. The M4 is the United States Military designation for the Stoner design with a shorter barrel and a collapsible stock. The Mk18 is the Navy's designation for an even shorter barreled rifle.
So the M4 is strictly a US Military Designation. and the M4 has full auto capability
AR-15 is the the original design and what most people call any rifle (but mostly Civilian) derived from that Eugene Stoner design
No the SIG M400 is not an M4.
As far as I know only Colt and Fabrique Nationale make M4s for the US military and there are no Civilian legal M4s because they were developed after the 1986 Full Auto registration cut off.
So the M4 is strictly a US Military Designation. and the M4 has full auto capability
AR-15 is the the original design and what most people call any rifle (but mostly Civilian) derived from that Eugene Stoner design
No the SIG M400 is not an M4.
As far as I know only Colt and Fabrique Nationale make M4s for the US military and there are no Civilian legal M4s because they were developed after the 1986 Full Auto registration cut off.
LocoGringo
Member
An M4 is a specific configuration of an AR-15. Most AR-15s that are claimed as M4s are actually Mforgeries. An M4 is a burst or select fire carbine length gas with a 14.5" barrel (I think) with a specially contoured barrel to allow for a M203 to be attached underneath the barrel. That's SBR length for civilians.
MachIVshooter
Member
M4 has 14.5" barrel and is select fire
M4gery has 16" barrel w/M203 cut and is semi-auto
Pretty much every other example is generically referred to as an AR-15 or just AR. Folks usually differentiate the larger variants by calling them AR-10 (technically correct only if an Armalite) or .308 AR.
M4gery has 16" barrel w/M203 cut and is semi-auto
Pretty much every other example is generically referred to as an AR-15 or just AR. Folks usually differentiate the larger variants by calling them AR-10 (technically correct only if an Armalite) or .308 AR.
I'll have to research it more later but I think the feed ramps and/or locking lug angle are different on the M-4 versus the AR-15. I've got an "iffy" computer right now so maybe someone else would like to have a go at researching the chamber configurations.
Bexar
Bexar
locking lugs are the same...except for specialty bolts like Knights Armament.
The M4 does have modified feed ramps...but for all intents and purposes...the M4 IS an AR-15. Civilian 16" collapsible stock carbines can have the same feed ramps as an M4...but they are not an M4
M4 is the US Military designation.
AR-15 has become a catch all term for any rifle derived from the Eugene Stoner design which was a downsized AR-10
Neo-Luddite
Member
AR-15 has come to be an overarching category for all similarly patterned weapons chambered in 5.56/.223 (although other calibers may labeled as such apart from 7.62 Nato, the AR-10).
Some weapons both military and civilian have been marked as such in both semi and select fire configurations. The notion that 'AR 15' necessarily denotes a semi-auto only weapon is untrue; many military M16/M16a1'a are marked AR 15. Although this is at least uncommon in later military versions of the rifle AFIAK.
The "M" prefix has been mostly a military designation--at least up until Colt won the right and has been able to market it's semi-auto carbine length mil spec pattern rifle as "M-4", and in fact marks the weapon as such.
This muddies the waters, but not by too much. For practical purposes, An M-4 clone, or M-4gery is a weapon equipped in a similar manner as the Colt 6920 in terms of specifics. This means a 16" barrel, removable carry handle, telescoping stock, and so on--a weapon that closely approximates the the M-4 in appearance and function, presumably in semi auto for most mere mortals.
And it is the sort of thing that will start shouting matches at the gun store.
Mostly it's all a matter of marketing if you ask me.
Some weapons both military and civilian have been marked as such in both semi and select fire configurations. The notion that 'AR 15' necessarily denotes a semi-auto only weapon is untrue; many military M16/M16a1'a are marked AR 15. Although this is at least uncommon in later military versions of the rifle AFIAK.
The "M" prefix has been mostly a military designation--at least up until Colt won the right and has been able to market it's semi-auto carbine length mil spec pattern rifle as "M-4", and in fact marks the weapon as such.
This muddies the waters, but not by too much. For practical purposes, An M-4 clone, or M-4gery is a weapon equipped in a similar manner as the Colt 6920 in terms of specifics. This means a 16" barrel, removable carry handle, telescoping stock, and so on--a weapon that closely approximates the the M-4 in appearance and function, presumably in semi auto for most mere mortals.
And it is the sort of thing that will start shouting matches at the gun store.
Mostly it's all a matter of marketing if you ask me.
Tommygunn
Member
Eeeeeyup. I have one so marked.
But as others have pointed out it is semiauto only.
Helluva nice carbine still.
Neo-Luddite
Member
Tommygun ~ I agree on the Colt quality. I bought our daughters each a Colt 6920 last year--the fit and finish was outstanding.
Yes, love my Colt M4/6920. It was a sub MOA rifle out of the box, bench rested. It has been 100% reliable with all kinds of ammo.
whenever the military decides to buy something, they get very specific about what set of features they want. They then attach a letter and number designation to it.
Let's say the military wants a new shoe string and I make them, so the .gov buys from me. They designate it K-55 (the old one was K-54). I make the government 10,000...and then make another 10,000 to sell to camping stores etc. Only the ones I sold to the government are K-55s. The other ones may be the same-damn-thing, but they aren't K-55s
Similarly....
The AR-15 is a type of rifle designed by Eugene Stoner and others, it has some design features and patents etc. There are some options that are not intrinsic to the design, like color, barrel length, etc...so there are a lot of different variants of the AR-15.
There is one specific set of features that the US army wanted, and they have chosen to call that specific variant the M-4. Key features of this are the barrel length, collapsable stock, ability to fire more than one round per trigger pull, a narrowing of the barrel to attach a grenade launcher etc. An AR-15 with basically the same feature set was sold to the Canadian Army, and they call it the C-8.
Let's say the NFA gets repealed tomorrow, and on Monday you run to the store and buy a full-auto AR-15....you still have an AR-15 and not an M-16...not unless some national guard gets permission to sell their old guns.
Note that many people have slapped M-4 on non-government rifles. But then I can make a super-tough computer case and call it M1 Abrams. I can name my dog M-4.
For a while bushmaster made a gun they labeled M-4, but then Colt got the rights to use M-4 on one of their rifles (a non-military gun) and so now no one else can label a rifle M-4. People/companies who build rifles that are SIMILAR to the military M-4 because the overall barrel+flash supressor length is roughly the same length as the barrel + flash supressor as the military M4. Of course most of these guns are semi-auto only, and may have other differences like a slightly thicker or thinner barrel, different stock, etc. Another term for these is 'm4gery' a play on the word 'forgery' because they look like the M-4 but as they are semi-only they are extremely different in function.
Let's say the military wants a new shoe string and I make them, so the .gov buys from me. They designate it K-55 (the old one was K-54). I make the government 10,000...and then make another 10,000 to sell to camping stores etc. Only the ones I sold to the government are K-55s. The other ones may be the same-damn-thing, but they aren't K-55s
Similarly....
The AR-15 is a type of rifle designed by Eugene Stoner and others, it has some design features and patents etc. There are some options that are not intrinsic to the design, like color, barrel length, etc...so there are a lot of different variants of the AR-15.
There is one specific set of features that the US army wanted, and they have chosen to call that specific variant the M-4. Key features of this are the barrel length, collapsable stock, ability to fire more than one round per trigger pull, a narrowing of the barrel to attach a grenade launcher etc. An AR-15 with basically the same feature set was sold to the Canadian Army, and they call it the C-8.
Let's say the NFA gets repealed tomorrow, and on Monday you run to the store and buy a full-auto AR-15....you still have an AR-15 and not an M-16...not unless some national guard gets permission to sell their old guns.
Note that many people have slapped M-4 on non-government rifles. But then I can make a super-tough computer case and call it M1 Abrams. I can name my dog M-4.
For a while bushmaster made a gun they labeled M-4, but then Colt got the rights to use M-4 on one of their rifles (a non-military gun) and so now no one else can label a rifle M-4. People/companies who build rifles that are SIMILAR to the military M-4 because the overall barrel+flash supressor length is roughly the same length as the barrel + flash supressor as the military M4. Of course most of these guns are semi-auto only, and may have other differences like a slightly thicker or thinner barrel, different stock, etc. Another term for these is 'm4gery' a play on the word 'forgery' because they look like the M-4 but as they are semi-only they are extremely different in function.
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Remington has a government contract for weapons, thus making them milspec, too.
One of the critical issues designating a firearm as .Gov property is the extensive documentation required by contract to prove the gun was built to the requirements outlined in specifications. This is where items like magnetic particle inspection of every bolt or the use of a specific steel is detailed.
The supplier has to include proof such things are in compliance for each and every weapon submitted under the contract. Here's the catch-22 - it's negotiated and changes as the weapons age and weaknesses are discovered. Then improvements are tested and those changes become part of the next contract, usually due to a "type" variation.
The military is currently looking at off the shelf parts for an improved "M4+" to be approved as a new type. This usually leads to a rush by the civilian market to include those items on a model to sell to those who want the closest legal version they can get.
A few years back when the M4 came out, M4geries sprouted up all over the market - with carbine gas and 16" barrels, trying to meet the specs and still be legal. The issue with that is carbine gas isn't the most optimum port location for a 16" barreled gun and demonstrates it by being overgassed and contributing to battering the bolt, having bolt bounce, and increased numbers of partially extracted cases that are still in the process of burning the propellant.
It was significant enough the civilian makers scaled back the emulation and started using "mid length" gas which was the more optimum port location for a 16" barrel. It reduced the overpressure and number of malfunctions due to it.
This is the kind of thing where the "M4" specification and the "AR15" have some divergence - and where the public goes wrong thinking they have to have as exactly as possible the military feature. In the case of the 16" it's not the best case option.
The quad rail got the same overemphasis - everyone demanded them on the M4geries, and now realize they are better off without them. An issue quad rail has 4 feet of rail space to just about mount any SOPMOD accessory wherever it might be needed - but the military doesn't really do that. Most units have only a few of them determined by their designated description of job duties - Infantry vs Signal, for instance, One may have an optic with 4x magnifier, laser designator, and light, the other a set of BUIS or red dot only. About 4 feet of the rail on the latter is completely unused, which makes for a heavier, bulky weapon. The quad rail is an institutional compromise to get any M4 made to be capable of accepting accessories - which most users don't have.
So we see with AR15s now, slick free floats with small detachable rails, and even non rail attaching holes in competing patterns. Another potential improvement that might make the M4+ list.
Overall, with some judicious selection of parts, acknowledging that the military specs are often a minimum standard at best, not the best available, an AR15 can be kitted out in a more reliable and even superior way to meet the demands of a specific set of tasks vs the M4 as a basic issue weapon.
Not what some would have come to conclude at first examination.
One of the critical issues designating a firearm as .Gov property is the extensive documentation required by contract to prove the gun was built to the requirements outlined in specifications. This is where items like magnetic particle inspection of every bolt or the use of a specific steel is detailed.
The supplier has to include proof such things are in compliance for each and every weapon submitted under the contract. Here's the catch-22 - it's negotiated and changes as the weapons age and weaknesses are discovered. Then improvements are tested and those changes become part of the next contract, usually due to a "type" variation.
The military is currently looking at off the shelf parts for an improved "M4+" to be approved as a new type. This usually leads to a rush by the civilian market to include those items on a model to sell to those who want the closest legal version they can get.
A few years back when the M4 came out, M4geries sprouted up all over the market - with carbine gas and 16" barrels, trying to meet the specs and still be legal. The issue with that is carbine gas isn't the most optimum port location for a 16" barreled gun and demonstrates it by being overgassed and contributing to battering the bolt, having bolt bounce, and increased numbers of partially extracted cases that are still in the process of burning the propellant.
It was significant enough the civilian makers scaled back the emulation and started using "mid length" gas which was the more optimum port location for a 16" barrel. It reduced the overpressure and number of malfunctions due to it.
This is the kind of thing where the "M4" specification and the "AR15" have some divergence - and where the public goes wrong thinking they have to have as exactly as possible the military feature. In the case of the 16" it's not the best case option.
The quad rail got the same overemphasis - everyone demanded them on the M4geries, and now realize they are better off without them. An issue quad rail has 4 feet of rail space to just about mount any SOPMOD accessory wherever it might be needed - but the military doesn't really do that. Most units have only a few of them determined by their designated description of job duties - Infantry vs Signal, for instance, One may have an optic with 4x magnifier, laser designator, and light, the other a set of BUIS or red dot only. About 4 feet of the rail on the latter is completely unused, which makes for a heavier, bulky weapon. The quad rail is an institutional compromise to get any M4 made to be capable of accepting accessories - which most users don't have.
So we see with AR15s now, slick free floats with small detachable rails, and even non rail attaching holes in competing patterns. Another potential improvement that might make the M4+ list.
Overall, with some judicious selection of parts, acknowledging that the military specs are often a minimum standard at best, not the best available, an AR15 can be kitted out in a more reliable and even superior way to meet the demands of a specific set of tasks vs the M4 as a basic issue weapon.
Not what some would have come to conclude at first examination.
OilyPablo
Member
Here is a helpful guide (and yes above most people have answered):
The happy switch is the only thing that really difers between an AR15 and an M4/M16 type rifle.
The original design uses a direct impingement gas system while some newer designs feature a separate movable gas piston to actuate the action.
The original design uses a direct impingement gas system while some newer designs feature a separate movable gas piston to actuate the action.
AR-15s were available with Happy Switches. The Air Force got a bunch of them before the rifles were given the official designation of "M-16". I shot a couple of them during my time in service. I shot them only during training and qualification and never in full auto.
The original design uses a piston located in the carrier. The original design is NOT a direct impingement gas system, a fact Eugene Stoner points out in his patent. Colt describes the system as direct gas.
"AR-15" covers a lot of territory. It's been used by Colt, as well as many others, to describe the whole family of weapons in a variety of configurations, from full length rifles to short barreled pistols and SBRS. "M4" describes a specific configuration of AR that Colt developed and sold to the military (Colt had recently lost the M16 contract to FN and needed something to get back in the game). Confusing? Not as confusing as trying to make sense of cartridge designations!
Back in the day, a technical article said the barrel length of the M4 was developed by reducing barrel length while keeping the front sight block at a location that would still allow the mounting of a bayonet while maintaining reliable function, then adding a half inch for insurance. The shortest length to meet that criteria was 14 inches. It became 14.5 inches with the added half inch.
To make the civilian M4 clones legal, the barrel was lengthened to 16 inches. The increased barrel length made it impossible to mount a bayonet, which critics of the time made a big deal of. I don't think any of the manufacturers (except maybe Colt) put much thought into refining the gas port size to compensate for the extra blow down time. The result was a bunch of over gassed carbine gas system civilian M4 clones.
Armalite chose to address the problem by developing the 16 inch middy. They did so to create a softer recoiling carbine. At the time, the marketing hype was that the new middy would allow the user to mount a bayonet to a 16 inch AR! That's how much influence the bayonet still exerted at the time.
The AR FOW has greatly matured in it's time, even more so in the last 5 years. Smart makers have a better understanding of how gas port size affects the operation of the AR and now offer rifles in all barrel lengths that shoot smoother and more reliably than ever, in all barrel lengths down to the shortest pistol/SBR, suppressed or unsuppressed. There are makers still stuck in the Dark Ages with their over-gassed systems but others have seen the light and moved forward
Quentin
Member
LocoGringo nailed it. The M4 is one specific configuration of the AR-15. There are countless other configurations of the AR-15 that are NOT the M4.
To whit, the M16, M16A1, M16A2, M16A3, M16A4, M4, MK4, MK4, MK12, MK18. In the past the XM177 and M231 port firing weapon.
Barrel lengths have been 20", 14.5", and 10.5", with some variants. But not 16". That is typically NOT issue, but seems to have the bulk of civilian sales due to NFA restrictions, as does 7.5" and other pistol barrels that are NFA legal.
Calibers as issued in the AR family include the AR10's .308, used by the Portuguese, 5.56, some 7.62x39 build by Colt for special operations, and three Mid East countries issuing SBR's in 6.8SPC. Not to forget the Brits ordered and bought more "AR10" versions from LWRC as semi auto sniper rifles, the L129A1.
The FNFAL was so obsolete it wasn't considered.
Barrel lengths have been 20", 14.5", and 10.5", with some variants. But not 16". That is typically NOT issue, but seems to have the bulk of civilian sales due to NFA restrictions, as does 7.5" and other pistol barrels that are NFA legal.
Calibers as issued in the AR family include the AR10's .308, used by the Portuguese, 5.56, some 7.62x39 build by Colt for special operations, and three Mid East countries issuing SBR's in 6.8SPC. Not to forget the Brits ordered and bought more "AR10" versions from LWRC as semi auto sniper rifles, the L129A1.
The FNFAL was so obsolete it wasn't considered.
I've seen M4's with barrels shorter than 10" that came with a can. I think the actual barrel was 7". And yes it was full auto plus it was mostly titanium. I got to hold it. I would have loved to get to shoot it. Heck I would have stolen it if I had the chance but considering it was a cop friend showing it to me that probably wouldn't have been a good idea. 
I'm not going to steal one but I've love to have one. That thing was about 4 lbs. too. Nicest firearm I ever picked up I guess and I've shot some nice stuff including full auto AR-15's. That M4 was a department gun of course. I wish I could buy one. It makes me want to become a LEO just so I could have access to the great toys they get and a budget for ammo that doesn't come out of my pocket. If it wasn't for all that other stuff they have to do...
I should have taken a picture is what I should have done.
I'm not going to steal one but I've love to have one. That thing was about 4 lbs. too. Nicest firearm I ever picked up I guess and I've shot some nice stuff including full auto AR-15's. That M4 was a department gun of course. I wish I could buy one. It makes me want to become a LEO just so I could have access to the great toys they get and a budget for ammo that doesn't come out of my pocket. If it wasn't for all that other stuff they have to do...I should have taken a picture is what I should have done.
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