The whole world knows the shape and sound of the US military's M-16 rifle. In service since the 1960's in one form or another, the weapon platform has seen use in battles across the entire globe. Not quite as well known is it's history, or the man mostly responsible for the M-16 with it's gas impingement form of action.

Eugene Stoner was working for
Armalite when he conceived the AR design of rifle. Through various trials it found it's way into the military inventory and first saw service in Southeast Asia. Amidst complaints of reliability issues, Stoner redesigned the rifle (the AR-18) and reverted back to the short stroke gas piston. The military decided to stay with it's M-16 investment, and corrected it's ammunition and training issues that were the root cause of the M-16's issues.
Over the years, Stoner's updated and improved AR-18 design found favor with arms makers outside the US, and Morphed into many well known military weapons of the century, although it never came home to the US military. H+K especially liked the design, and borrowed many parts of it for their battle rifles. The Stoner AR-18 shadow falls heavily on the SA80, the SAR80, and the G-36.
On the home front, both the Armalite company and the AR-18 patent saw new owners and countries. The AR-18/ AR-180 itself became as much a collectors rifle as a shooter. Sometime around the year 2000 a reformed Armalite redesigned the AR-180 again, into the
AR180b, and began production.

The AR180b combined the best features of the AR-15 and the AR-180 into one lightweight and dependable package. Never designed for full automatic fire, it's sales were aimed at the civilian and police markets. Introduced at a price several hundred dollars less than the competition's AR-15 rifles, it should have taken off well, and would have if dealers had not universally raised the price to AR-15 levels as it hit the store racks. After all, it was black and looked like an AR-15, so why shouldn't they get full AR retail for it?
Armalite still makes the AR180b, and it has a small but loyal following. Knowledgeable AR180b owners are now having giggling fits as they read articles loudly announcing new short stroke piston kits for the AR-15/M-16 rifles, the AR180b having had this reliable action from the beginning.

The AR180b weighs in at just over six pounds, and that's with a full length barrel. Much of the weight savings came from making the lower receiver of high strength plastic. For any normal service the receiver is indestructible, and will survive things that would destroy an AR-15. While redesigning the lower unit, Armalite built it to use standard AR-15 magazines and an AR-15 trigger group. This allowed the use of all the surplus magazines on the market, and also made repairing any trigger issues a breeze, with parts available from a dozen sources.
The upper unit is where things get different. A shooter need only use a cartridge to depress a plunger at the back of the action, and it will tilt apart similar to an AR-15. The similarity is only cosmetic, as everything else is different. Very different.
The bolt does not ride against the receiver, excepting a stud used to rotate the bolt head. This makes the action very reliable in dirty conditions, having room for debris to fall free. Instead, the bolt rides on two long rods, which also guide the recoil springs. Both rods and springs are contained in the receiver, unlike the AR-15. This means the AR180b (and AR-180) can have a real folding stock.

The bolt is acted on by a short stroke gas piston riding above the barrel under the upper forearm. Having removed the bolt and guide rods, the forearm can simply be lifted off and the entire gas system cleaned and serviced. The entire field stripping can be done with no tools, and achieved in under a minute. Reassembly is slightly more fiddly, with care needed to properly insert the bolt into the receiver, but can still be done in a about a minute.
Unlike the AR-15 type rifle, with the AR180b no gas enters the action. This increases reliability and makes cleaning a breeze. No more digging hardened carbon out of the bolt head!
Another major difference incorporated into the AR180b, the charging handle is a knob directly inserted into the bolt. This greatly simplifies working the action, and does away with the need of a forward assist. The action can also be manipulated with the rifle shouldered, unlike the AR-15 which requires a charging handle be pulled back right where a shooters face would be.
One point the AR-15 does have over the AR180b is the bolt release. The Armalite does not have one, and this makes shooting single rounds problematic. The AR180b won't be a contender in high power competition any time soon. On the other hand, reloading the chamber after inserting a full magazine is fast and instinctive, and many find it faster than hunting the small lever on the side of the AR-15.

The AR180b does not come with a flash 'suppressor', the ATF having deemed them entirely too evil for normal humans to own. Instead, it has a muzzle break that is reasonably effective. It's machined as part of the barrel, and some folks have a gunsmith replace it with a traditional flash suppressor. Likewise it does not come with a bayonet lug, the ATF having determined that drive by bayonetings are too common to allow the lug.

One unique feature of the AR180 and 180b is the built in proprietary scope base. It's a wedge-like affair welded to the top of the upper receiver. While a little strange, it's extremely effective. An optical sight can be mounted and dismounted easily by hand, returning to zero each time. The downside is availability of scope mounts to fit it. Only Armalite and a few aftermarket specialists make the spring loaded mount it requires.
One company,
StorkWerkz, makes a high quality mount that in turn has a standard rail on it. They also sell replacement forearm sets of machined aluminum with multiple rails, just in case the 'robowarrier' image is desired.
My experience with the Armalite AR180b has been a good one. Over the years I've owned a few AR-15 platforms (all Colts) and each has fallen by the wayside, traded or sold. The Armalite is unlikely to suffer that fate, appealing to me more than any of the other black rifles ever did. With an Eotech 512 mounted on a StormWerkz mount, the rifle is boringly easy to shoot well. Hitting the target rapidly is a snap, as the rifle is light and recoil is minimal.

I ran across this rifle by sheer luck, unfired and in the original box for only $575. That is, strangely, the original suggested retail price as well. The rifle now retails for over $850, if you can find one. I consider myself a happy Armalite owner.