SR-15 FF RAS vs SR-25 FF RAS


SOPMOD M4 Carbine
KAC Free Float RAS
KAC 2-Stage trigger
Knight's SR-47
Knight's SR-25K
M5 RAS on AR-10
AR-10 Rail FF Tube
M5 RAS Dissipater


SR-15 FF RAS and the AR-10

I have been interested in the SR-25 with a particularly strong interest in the SR-25 Match rifles with the Knights Free Floating RAS system for quite some time as I describe in this page.

It should be clear that Knights Armament has begun supplying their SR-15 Free Floating RAS tubes to the general public as they are appearing for sale on many internet forums and at gun shows. Even better is the fact that Wes Grant at M.S.T.N has been selling these Free Float RAS tube assemblies in several lengths that I describe in considerable detail in my Knights FF RAS Assemblies page.

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)

I had purchased a LONG Free Float RAS that I originally planned to use for an AR-15 project. With that said, I recently read a post on by an individual that uses a long KAC FF RAS from an SR-25 on an AR-10. He stated that he was able to mount his SR-25 FF RAS on his AR-10 by simply exchanging the SR-25 barrel nut with an AR-10T barrel nut. He further advised that the opening in his SR-25 FF RAS collar is the same size as the Armalite AR-10T collar. With this information, I E-Mailed Armalite and confirmed that the collar that mounts the fiberglass Free Float tube to the barrel nut is the same for both the AR-15T and the AR-10T and that the only part that was different between the AR-10T and AR-15T free float assemblies are the barrel nuts.

Following is a picture of a short Knights FF RAS tube that is labeled to show what these parts look like. Regardless of length, all KAC FF RAS tubes that I have seen come with the parts shown below.

  1. Handguard Collar (Attaches Forend tube to barrel nut)
  2. Knights Short FF RAS forend tube
  3. Knights SR-15 FF RAS Barrel Nut
  4. Two alignment pins

Given the discussion above, I decided to buy an AR-10T barrel nut from Armalite. This part sells for $15 (part #EA-1146) and I received it March 22, 2002.

Well, to make a long story short, the SR-25 FF RAS is significantly larger than the SR-15 FF RAS tubes. It was my previous understanding that these SR-15 FF RAS and the SR-25 FF RAS used the same tubes which is clearly not the case as I have been advised that the SR-25 FF RAS as on a Mk 11 Mod 0 system measures 2.65-inches from the top to bottom surfaces of the rails and 2.45-inches from side to side of the rails. The SR-15 FF RAS tubes on the contrary measure 2.318-inches from the surface of top rail to surface of bottom rail and 2.184 from the surface of the left rail to surface or right rail. There are also differences in the SR-15 and SR-25 barrel nut and handguard nuts.

I do not see any way that I will be able to use the AR-10T barrel nut with an SR-15 FF RAS tube for several reasons.

1. The SR-15 FF RAS is to small to accept an AR-10T barrel nut.
2. If an AR-10T barrel nut is modified to fit, there will not be room for the gas tube.
3. The top of the AR-10 flattop rail will not align with the SR-15 FF RAS rails.

At this point in time I do not see any solutions to these problems and I have come to the conclusion that an SR-15 FF RAS can not be used with an AR-10. It is my understanding that Armalite as well as some others are working on MIL-STD-1913 rail systems for the AR-10/SR-25 and unless Knights is someday willing to sell us the SR-25 FF RAS tubes, this may well be as close as we can get.

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This site was last updated 01/07/04