The Free Floating RAS system combines the basic utilitarian nature of the traditional RIS/RAS with a Free Floating Match style tube that allows all the weight of the accessories that are mounted on the forend tube to be supported by the receiver at the barrel nut so as to not cause any flexing of the barrel because of weight
transferred from the forend.
The SR-15 FF RAS and the SR-25 FF RAS tubes are significantly different tubes and are not
interchangeable as I discuss on this page.
In general the SR-25 FF RAS is significantly larger than the SR-15 FF RAS making
it such that the SR-15 FF RAS will work with any AR-15/M-16 clone while the
SR-25 FF RAS will work with the SR-25 and AR-10 weapon systems.
To my knowledge there are five basic FF RAS tubes available for the SR-15. Following is a picture of the three FF RAS forend tubes that are most often seen for sale to the general public.
Left: Short M4 Free Float Carbine R.A.S. (KAC P/N: 20208)
Middle: M4 Carbine RAS Assy "MEDIUM" Length KAC P/N: 20214
Right: M-4 Match/Sniper Free Floating R.A.S. Forend (P/N 99266)
Short M4 Free Float Carbine R.A.S. (KAC P/N: 20208)
Short FF RAS
This is the FF RAS that has been referred to as the "short" model in the past (KAC P/N: 20011). This FF RAS is the same length as a conventional RIS or RAS as it measures 6 7/8-inches (6.197-inches) in length without the barrel nut attached. This short length is a direct replacement for a Colt, Bushmaster or other short carbine length forend grip and would allow the use of the full front sight on these weapons. Because the FF RAS does not clamp onto the barrel in any way, this tube can be used with any barrel profile including both lightweight and bull barrel profiles.
M4 (FF) Rail Assembly System (KAC P/N: 991687)
Medium FF RAS on a Suppressed 9mm upper
This is a rather uncommon tube based on the fact that I have only ever seen one of these which I quickly decided to purchase. This FF RAS is generally described as the "MEDIUM" model and forend tube itself is 7 5/8-inches in length without the barrel nut attached. This tube is approximately the same length as the Mid-length carbine sold by Armalite and Rock River Arms and I believe that this forend tube would allow an owner of one of these Mid-Length uppers to use this FF RAS with the normal front sight/gas block unlike the medium-long FF RAS that follows. I have one of these unique Medium Length FF RAS tubes that I am planning to use for a 300 Whisper project that I
describe on this
M4 Carbine RAS Assembly "MEDIUM" Length (KAC P/N: 20214)
Medium-Long FF RAS
This is the FF RAS that has been referred to as the "medium-long" model in the past (KAC P/N 99166). The forend tube itself is still labeled P/N 99166 according to MSTN who sells these tubes on a fairly regular basis. This FF RAS tube is 10-inches in length (9.349" of rail surface) without the barrel nut attached. This tube is considerably longer than the Mid-length carbine sold by Armalite and Rock River Arms and is often used with a special low profile gas block from Knights armament that allows the gas block to sit under the FF RAS tube. This FF RAS combined with a low profile block can be used with both standard and mid-length carbine uppers as well as just about any blow back cartridge such as a 9mm or 22-LR conversion were a gas block is not required. This allows the user of this FF RAS to use a rail mounted flip up sight or to not use any forward sight with a greater area of Mil-Std-1913 rails to which accessories can be mounted.
M-4 Match/Sniper Free Floating R.A.S. Forend (KAC P/N: 99266)
Long FF RAS
This is the FF RAS that has been referred to as the "long" model in the past (KAC P/N 99167). This FF RAS tube is 12-inches in length (11.27" of rail surface) without the barrel nut attached and is a direct replacement for the A4 length grips used on a 20-inch M16A2 upper. These forend tubes are used on Knights full sized SR-15 Match rifles as well as special forces sniper rifles around the world including the SR-25 Mk 11 Mod 0 and SPR-B light sniper rifle. Early versions of this tube were labeled "M-4 SNIPER R.A.S." which apparently upset Knight’s legal department so all later models were marked "M-4 MATCH R.A.S."
KAC FF RAS "MODULAR RAIL EXTENSION" for the Navy SEALS
I have been advised by
Wes Grant at M.S.T.N. that Knights Armament has made this unique FF RAS for the Navy Seals. I was able to handle one of these FF RAS tubes at a
gun show this weekend (3/24/02) as Dennis Todd had one of these tubes. This FF RAS is 12" long with a unique tube that has a section that can be removed such as to allow the tube to be used with a traditional carbine length barrel.
The removable section contains MIL-STD-1913 rails and when this section is mounted in this FF RAS the tube works just like the LONG FF RAS described above. When this section is removed, the FF RAS has a hole on the top and bottom rails that allow a Carbine length Front sight to slide inside of the FF RAS. Then a shorter piece is mounted forward of the front sight. Wes describes the advantage of this FF RAS as "it provides more real estate for mounting the AN/PEQ-2 IR Target designator/illuminator forward on the left or right side of the forend tube (on a M-4 Carbine upper)." Wes has also advised that the limited numbers of these that are available are from a Navy Seal contract overrun.
The SR-25 FF RAS Tube
Full Length SR-25 Free Floating R.A.S. Forend (KAC P/N: 98513)
SR-25 FF RAS on MK 11 Mod 0 Sniper Rifle System
This is the FF RAS that is used on the SR-25 Match Rifles as well as special forces sniper rifles around the world including the SR-25 Mk 11 Mod 0. This FF RAS tube is 12-inches in length without the barrel nut attached. This FF RAS is currently only availed through the purchase of an SR-25 Match or other special edition weapon.
Carbine length SR-25 Free Floating R.A.S. Forend
PRE-BAN SR-25 RAS CARBINE, Medium length FF RAS
This is one of a hundred unique pre-ban SR-25 carbines that Knights sold in early 2001. These are unique guns complete with
collapsible stock, flash suppressor, and this unique medium length carbine FF RAS tube. I was advised by
John Carney that the FF RAS on the SR-25 RAS Carbine is 9.25" from the front of the collar to the front of the rail. This FF RAS tube does not have a KAC part number on it but it looks to be the same length as the medium-long FF RAS for the SR-15 as described above. Following are three additional images of the FF RAS on an SR-25 RAS Carbine.
Post-Ban SR-25 RAS CARBINE, Medium
length FF RAS
CLICK to see Left side of this tube.
CLICK to see Right side of this tube.
I am trying to get more details on the SR-25 FF RAS tubes (as well as any SR-15 tubes that may have been missed) at this time. If anyone can provide any additional details, please E-Mail me and I will add the information here. Specific information I would like to add includes rail markings as well as any unique lengths that may be available. If anyone has an SR-25 FF RAS that is not mounted to a weapon, please E-Mail me so that we can get a more accurate description of the SR-15 vs. SR-25 FF RAS tubes. THANKS!