I know its a rifle that is/was made by colt, and its a derivative of the M-16A1 and has neither selective fire or burst, chambered .223/5.56 and comes with a diverse collection of accessories.
So you would think that I have at least a general idea of what this firearm is, but ya know facebook, and seems springfield armory brought out their AR-15 and called it the saint. I make no bones about this firearm, but its missing the handle thingie that to me, defined both the 15 and the 16.
Ruger makes a similar weapon, and yet they named it the mini-14, as well chambered for .223/5.56 and its ability to mount what ever gadget that makes a minute man happy. So what is it that defines an AR-15, its almost like saying a ford camaro. Is it a patent that as long as its licenced, any company can call their firearm, an AR-15.
The original AR-15 was made by Armalite, and it was the weapon that the M-16 was based on. The Colt version was in turn based on the M-16. Apparently, anybody can call it an AR-15, and do.
As **Chefguy *said, a version the Armalite AR-15, as adopted with modifications by the US Military in the 1960s, was given the designation M-16. From there on various upgrades have been created for the services as M-16A in successive order. Colt, contracted supplier of the M-16, bought from Armalite the rights to both M-16 and AR-15 and then sold AR-15s, based on the military-adopted version, to the civilian market by building in a semiauto-only firing mechanism, and this established in the general public that the AR-15 was the “civilian” version of an M-16.
(Note: The AR-15 was a design for a lightweight assault rifle by Eugene Stoner. The Ruger Mini-14, though, actually has a Garand-based operating mechanism – what is meant when it is said to be “functionally the same weapon” is that even though the standard Mini-14 [not the pimped out tacticool variant] looks like a staid old sporter rifle, its guts are still based on a tested military design and it fires the same ammo as an AR-15.) [ETA - ninja’d by running coach]
What you mostly have around in common media today is a matter of “brand erosion”. Only Colt can really sell it specifically under the AR-15 trademark, but the patent on the mechanism and system design has long since ended so anyone can make an “AR-15 Variant” or “AR-15 class” weapon (think “PC Compatible”)
Then also there’s how people will call “an AR-15” anything that just has a largely the similar form factor and/or similar functional characteristics as the classic AR-15/M-16 family. Just like they’ll call “an AK 47” anything with the curved mag and the sight above the muzzle that even remotely looks the part whether or not it has a Kalashnikov-design action.
“AR-15” is a registered trademark of Colt Defense LLC. Not sure why they aren’t as seemingly militant about hammering unauthorized use of the mark as other TM holders.
The main patents for the AR-15 direct impingement gas design have long expired. Still, they may provide some insight as to the heart of the design. 2,954,424 is for a “Gas Operated Bolt And Carrier System,” invented by Eugene Stoner. IIRC, the AR-10, chambered in 7.62 x 51, came before the AR-15. Nowadays, one can get AR-15s in a variety of cartridges, from the original 5.56 x 45 and .223 Remington, to 6.8mm SPC and 6.5 Grendel to behemoths like .458 SOCOM and .50 Beowulf.
The Mini-14 is more akin to a Garand or M-14 than it is the direct impingement system of the AR-15/M-16/M-4. Of course, there are now piston-operated AR-15s, which muddies the water even more.
Not that I am an expert, but IMHO what makes an AR-15 is: direct impingement gas system allowing for semi-automatic operation, detachable box magazine, straight line stock with recoil buffer. The carry handle is not necessary.
To add to the extensive answers already, there are elements of the M-16 design that pretty much define a rifle as that general type.
In most rifles, repeating or bolt action, the bolt (which carries the full force of the ammunition at 50,000 psi) locks into the receiver. So the receiver is a very important part and cannot be compromised.
The M16 does not do that. The bolt locks into an extension of the barrel, so the barrel and extension carry all the force. The receiver is the most complex, expensive part of most firearms, but in the M16 it is just an aluminum piece that carries the parts. It can be cast, forged, machined from solid on your backyard milling machine or made in a 3D printer. It can be made of plastic (and ***has ***been), carbon fiber, fiberglass. Hell, it could probably be made from ice (well, at least Pykrete). Try that, Mythbusters!
The Ruger, as was pointed out, is not like this, it is just a conventional rifle strongly based on the M14. In fact, that is why it is called the Mini-14.
I agree with the answers summarized as, if it has the AR-15’s direct impingement gas system with same/similar bolt, bolt carrier, connection to the barrel extension, lower receiver and trigger mechanism, it’s basically part of the AR-15/M16 family no matter who makes it or variations in the upper receiver, barrel length, stock or external cosmetics. And it doesn’t have to be 5.56mm.
The close judgment calls tend to have originated on the military side where some weapons have included some of those things but not others. Like for example the Heckler and Koch weapons which adapt a gas pushrod rather than direct gas tube to an otherwise ‘M16 family’ type weapon. Similarly the revived AR-10 7.62m type weapons often have M16 components, and more closely connected to the AR15 family than the original AR-10 of which the AR-15 was a scaled down version with but with no parts in common. 9mm submachine gun versions of the family look externally very similar, but have a simpler plain blowback operating mechanism inside.
And just to clarify the original relationship v OP comment
“[the AR15 is] a derivative of the M-16A1”, not so. AR-15 originally meant the military selective fire weapon. When it was first adopted by the USAF in 1962 it actually carried the military designation ‘Rifle, AR-15’. The Army designated it M16 around the time they decided to procure it in quantity, though still not officially adopt it as ‘standard’ (and replace the M14, that only happened later) in 1964. The same year Colt introduced the commercial Model 6000 AR-15 SP1 which was a semi auto version of the military AR-15/M16, not the M16A1. Commercial AR-15’s didn’t adopt the telltale recognition feature of the M16A1, the bolt forward assist, until much later. However Colt commercial AR-15’s generally used parts from whatever version of the M16 was current so generally evolved along with it. For example Colt commercial AR-15’s became M16A2-like in various respects in the era of that weapon (heavier barrel, different rifling etc).
ArmaLite was a division of the Fairchild aircraft company. Its president at the time was a gun nut and he persuaded the other Fairchild directors that the Stoner design was worth making and would be a moneyspinner. After he left, Fairchild sold the apparently unprofitable ArmaLite AR-15 design to Colt. Although ArmaLite produced several designs, the AR-15 one was the only one that eventually became commercially successful.