The M-16 series of weapons first originated at the AR-10 in the 1950s, but have undergone a steady progressive evolution for nearly half a century. In every recent American military conflict, large and small, the M-16 series has never been absent. As one of the most representative assault rifles of the 20th century, the M-16 has undergone a dizzying number of various modifications and improvements.

In more recent years, there has been a significant increase in the development of military small arms. Most specifically, there has been a trend of increasing Modularity: the ability to freely and easily modify a weapon to suit the specific needs and enhance the tactical flexibility of an operator.

One company whose name is almost synonymous with weapons modularity would be that of the Knight's Armament Co.
The Knight's Armament Co. is based in Florida, USA, and has been working on a "Modular Weapon System", or MWS, for the last decade for primarily US military use. The Knight's MWS development focuses mostly on the M-16 and H&K families of weapons. One identifying aspect of the MWS is the familiar railed hand guards, known as the Rail Interface System, or RIS. The four rails of the RIS adhere to the American Military standard MIL-STD-1913 regulation, and can easily accept all standardized accessories on the market. Tactical lights, Red dot scopes, Laser aiming devices, Bi-pods, Vertical Foregrips, etc¡K In recent years, various components of the MWS have already been adopted by many American Special Forces units due to the inherent advantages of having such tactical flexibility in one's weapon.

The average M-16 rifle can be given MWS capability by simply removing the stock plastic hand guards and replacing them with RIS hand guards. The advantages of doing this would be the ease of installation and low cost, while also retaining the ability to change back to stock hand guards at any time.
However, the disadvantages would be that the weight of the RIS and any extra attachments would make the front end of the weapon heavy, and adversely affect accuracy. And over time, with heat generated from real M-16s (or inherent weakness of the Tokyo Marui barrel at stress points) the barrel's shape may be affected by the RIS and components, and result in a further loss of accuracy.
To deal with these issues, the people at Knight Armament developed the Free Floating Rail Adaptor System!

Compared to the RIS, the Free Floating RAS has a number of advantages:

As you can see, the Free Floating RAS is one of Knight's sharpest creations yet.

In the real gun market, the RAS has been produced for two main product lines, the SR-25 and SR-15, with the SR-15 retaining the M-16 series original 5.56 caliber, while the SR-25 adopts the 7.62 caliber. The only noticeable external difference would be from the lower receiver's magazine well, and the size of the magazines.
In the airsoft market, because of the lack of a 7.62 caliber stoner rifle, and in the interests of realism, only the SR-15 can be reproduced. Currently, two airsoft companies, Guay Guay, and GB-Tech have produced a SR-15 conversion kit. Given the competitive nature of the airsoft market, I will analyze the various differences between the two kits for you!
 

GuayGuay	GB-Tech
Size, Color, and Feel
The G&G produced SR-15 RAS is significantly different from the real version, with the total RAS, as well as securing ring at the end of the RAS being longer than the real version. From these differences, it's not difficult to identify this version from the real version. The entire RAS, from securing ring, barrel, to front sights have been treated to have a flat black color. This is significantly different from the Knight Armament version. In terms of feel, G&G has kept the clean cut, and well machined lines of the RAS, and because of this, some corners seem to be "sharper", while the Knight's version under goes a process which makes the angles of the RAS rails slightly duller, and does not "bite" into the hand. In terms of weight, the G&G kit weighs about 1170 grams.	The GB-Tech produced kit is much closer to the dimensions of the real SR-15 RAS than the G&G produced one, though the securing ring, like the G&G version, is a bit longer than it is supposed to be. In terms of color, the securing ring is a bit too gray, but the total product does not deviate too much from the real RAS. In terms of feel, the GB-Tech produced kit is much too rough, and has about the same grade as very fine sandpaper. The feel is much rougher than the G&G version, and this isn't too great for skirmishing, and may even hurt the hand in some conditions. In terms of weight, the GB-Tech kit weight at 800 grams.
The RAS main body, the rails, the RAS securing ring, Gas Valve, and front sight.
Aside from the "look" of the product, everything else is almost completely off the mark. The RAS main body is too thick, and worst of all, the rails are not of the American Military Mil-Spec, and hence there may be issues in mounting some accessories. As to the front sight and gas valve, they are absolutely nothing like what Knight produced.	The GB-Tech Version unfortunately, also does not have real sized dimensions. The RAS main body is too thick, and the rails are too thin, and also do not adhere to US Military Specifications. The RAS's securing loop, aside from being slightly longer than the real version, also have different texturing. The RAS's Gas Valve is a slightly higher than the real version, but the plain addition of one does make the GB-Tech version slightly more realistic.
Engraving.
The Knight RAS has these following engravings   Left side: KNIGHT'S ARMAMENT CO. VERO BEACH , FL. (561)562-5697   Right side: M4 SNIPER R.A.S. - PN: 99167 Notice in the picture to the right, that the "R" letter has a curvy end. Interesting eh?
The G&G version uses completely original engravings, which are nothing close to the real version.	The GB-Tech version uses the correct letters, but there are some differences. The Abbreviated "FL" lacks a period behind it, for example. Because of these small differences, it is presumable that GB-Tech used photographs for reference. Also, GB-tech is unable to reproduce the same lettering used by Knight. : )
Method of Production
The real RAS's method of production is quite unique. The real steel version is produced with direct welding of parts, and there are 8 clearly visible weld marks on the RAS. On a side note, the machining of the securing ring is very precise, allowing from the RAS to smoothly connect to the weapon itself. The materials of the Knight produced RAS are superb. I myself tried to scratch the RAS with my house keys, but I noticed that it was my key which was damaged in the experiment. Please note the copper of my key on the RAS, while the RAS doesn't even have a scratch!
The G&G version is constructed entirely out of Aluminum Alloy, and is CNC machined. Interestingly, there are seemingly useless holes machined onto the four rails. : )	The GB-Tech version's construction and machining is exactly like the G&G version's, and though isn't too shabby, it unfortunately passes up on the opportunity to replicate the real version.
Installation process: reliability
When I first handled the Knight produced product, I was curious as to how the RAS would be secured, and create a Free Floating effect. The Knight's design is so simple it blew my mind. By using only four components, the RAS secures itself and the rifle barrel onto the upper receiver. No matter how you secure it, the four rails are amazingly always horizontally and vertically aligned with the gun!
The G&G produced version does not utilize the same retaining system like the real RAS, but is quite simple. The barrel is first put in place, and the then the RAS can be locked into position. However one feature that didn't translate into this airsoft version is the ability for the RAS to remain vertically and horizontally in place no matter what. You need to make sure the rails are aligned before you can secure the RAS. I noticed that there are three numbers engraved on the front site, and on the barrel of the weapon. After brainstorming the meaning of these numbers I later figured out that these numbers are meant to be aligned, so that the front site too is correctly aligned. But because the aluminum of this kit isn't too great, the accuracy of this method may degrade over time with use. Because this kit uses a two part gun barrel, and is unable to be truly free floating, the front sight connects to the RAS, which in turn supports the barrel. GuayGuay: parts diagram   Numbers	The GB-Tech version of the RAS has the most complicated securing system of all three. The base of the barrel replaces the Tokyo Marui barrel base, and four components are needed to create the barrel (oh my¡K). Because of this, this barrel is unable to be truly free floating, and like the G&G kit, the front sight post/Gas Valve has to support the barrel by connecting to the RAS. I personally believe that a simplification of things would be better. The more complex a gun is, the more problems you will encounter on the field. From the first MGC Glock ten years ago, to the ones you find today, there has been a steady simplification of "Blow Back" technology. The importance of KISS, Keeping it Simple, Stupid can even be borrowed from real steel weapons technology. KISS has definitely played a huge role in the evolution of weapons in the 21st century. : ) GB-Tech: parts diagram
In Conclusion
Though both companies claim to use real steel dimensions, and promise functionality, both finished products do not live up to their promises. However, with GB-Tech being able to produce an entire kit just from photographs, I believe they deserve a "Good Hearted Try" Award! As to G&G's version with the most whack "real steel dimensions" I have ever seen, I believe they deserve a "Most Creative Award"! : ) Ok, ok, I'm just kidding with ya. It would seem that every domestic airsoft company (Taiwanese) recently has been promising "real steel dimensions", but as a wary consumer, I suggest you keep you eyes and ears wide open before you buy! :P

Additional Pictures

A comparison between the G&G and the GB-Tech versions

Both versions of the Airsoft RAS used aluminum to create their forward gas valves, but Knight used steel.

GB-Tech's multi stage barrel. The purple arrows point at connecting points.

The real deal Knight's RAS