I have a Knights Armament M-4 RAS (Not free float) on a 9mm AR.  I plan to mount an optic (aimpoint or reflex) to the RAS itself.  What deviation should I expect as the RAS is, of course, not as stable as the flattop. Since it is 9mm I do not expect to shoot at ranges greater than 100 yards or so.  Comments?  Are the advantages of greater eye relief outweighed by the less stable mounting platform of the RAS?

On a Colt 9mm AR, the M4 RIS might be a better choice than the RAS.  This is because the RAS has a two-pronged "spacer" at the front that is looking to rest on the .840" diamenter of an A2 type barrel, just behind the forend cap.  Your 9mm is probably much smaller in diameter than that, so the RAS is most likely resting on the two little side tabs of the forend cap.  This is ok as long as the cap is really tight, can you rotate it any?  Is this where the "play" in the RAS is most apparent?
ColdBlue sends...

nope. Can not rotate it and the cap seems quite tight.  However, I did have to remove the little tab portion in the front of the RAS to fit the larger profile barrel.

We need to talk.  All the colt 9mm barrels I have seen are smaller in diameter than .840.
You should not of had to remove the two fingered spacer, as this is what supports the front of the RAS.  If this is in fact the case, we need to trade you out for a RIS that "pinches" the forend cap for stability.
But the real test is does the rail support your optic with the accuracy potential of your weapon?
As always, "if it ain't broke, don't fix it".
Give me a call:
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Dave Lutz is ColdBlue at KAC...

I like mouting a reflex as far forward as possible.  I "Feel" like I can aquire targets faster with it mounted at that position.  YMMV

I currently have an Aimpoint Comp M forward mounted on a KAC RIS.  Like the shooting conditions you describe, I usually do not shoot it over 100 yards.  Using the ARMS 22M68 mount, I have had no problems with repeat zero, and can always count on being on target when I go to the range.   Hope this helps…

[img]http://photos.ar15.com/ImageGallery/IG_LoadImage.asp?iImageUnq=1192[/img]

I have the exact same settup M4-a3.  Sory for the basic question but what is the differencence between the RAS and the RIS?  I had to remove the center front sprin/tab of my RAS to fit the 9mm barrel.  That is, I removed the compnents held in place by the front roll pin.  
M4-a3 what sort of group can you shoot with the 9mm at 25 yards?  that is, is there much deviation?

Thanks again..

I can pretty consistently shoot 1” groups at 25 yards.  As far as the difference between the RAS and RIS, the RIS has a screw in the top portion of the handguard, which tightens a clap that holds the handguard to the forend cap.

thank you.  Apparently, I do then have the RIS.

RIS secures to barrel assy via a screw activated clamp located in the top rail at the front.  The clamp is first located so its ON TOP of the forend cap's folded over edge, and the rear of the RIS lowered into place.  The screw is then tighten to lever the clamp down to like "pinch" the edge of the forend cap.
RAS is completely reversed.
First off, the "tab" that sticks out the front of the ras GOES UNDER the folded edge of the forend cap. Secondly, remove the 1/8" allen screw at the rear of the RAS, then as the rear is then lowered, the clamp hangs down and straddles the gas tube, makes contact with the barrel, and is finaly camed under the lip of the barrel nut.  The 1/8" allen is then re-installed.  (see quarterbore's great site for more).
Before you install a RAS, look under the front end.  There is a leaf spring-loaded two-fingered "spacer" whose ends are concave.  These two concave fingers are intended to rest on the mil-spec barrel diameter of .840 that is located directly behing the handguard cap under the handguards.  9mm barrels don't have this .840, they are usually much smaller in diameter, so the spring loaded front spacer of the RAS will serve no purpose.  That is why i recommend the RIS for the 9mm, and even early AR barrels of the pre-A2 smaller barrel diameter.
ColdBlue sends...  

Ok.  After further review of theses posts I do have a RAS.  I removedd the leaf spring-loaded two-fingered "spacer" from the front (by the front sight tower)by removing the roll pin holding it in place as it would not fit over the barrel that I have (american spirit arms heavy barrel 9mm).  The front tabs which are part of the RAS itself still fit under the front cap.  
The clamp at the rear of the RAS is still in place and engaged the lip of the barrel nut as described.
I was just concerned that by removing the leaf spring-loaded two-fingered "spacer" from the front of the RAS it would make the RAS less stable.  Although perhaps theoretically true, removing it did not seem to greatly effect stability of the RAS,  Moereover, it matters not as the RAS would not otherwise fit the barrel.  

Thanks, now I have the complete picture.
I am glad it worked out for you.  You actually came up with a pretty good idea, .i.e., removing the leaf spring to allow the two fingered spacer to move further out of the way, thereby allowing a larger diameter barrel than it was designed for fit.
What I am wondering now is that if you found a drum sander about the diameter of your barrel, could you open the radius's of the fingers enough to allow the spring to go back it place?
By the way, this would be an improvement even it the spring is not re-installed.
However, next time just go for the RIS model, as it is more forgiving of non.milspec dimensions.  
ColdBlue sends...

Yes.  I suppose I could just modify the the two fingered spacer to fit the barrel.  I will give it a try.  It certainly would not hurt.
thanks again.

I opened up the two fingered spacee with a dremel tool and reinstalled the ras witht the front spring in place.  All is well now.  Not sure if the RAS is much tighter since it was pretty tight before -- however, it certainly could not have made it looser.  I also put a roll poin in the gas tube opening in the front sight which gave the two findered spacer another contact point(s).
thanks again.