Since the handguard cap is no longer being used, there is a small gap directly behind the FSB (see arrow).  Before placing the pins back into the FSB, I want to know if I can get rid of this tiny eye sore.  Is there a spacer or washer that can be placed to fill that little gap?  Thanks for the help guys.

If you fill that gap, won't you be defeating the purpose of a free floating rail if whatever you stick in there touches both the rail and the FSB?

ETA: From the little screen on my BlackBerry at least, it looks like your FSB isn't quite seated all the way back. It appears to be slightly forward of the thicker shoulder of the barrel.

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Sorry, I think I didn't explain it very well.  The gap is there because the handguard cap that was once there is no longer there.  The FSB is perfectly lined up, if I move it farther back, the pin holes will not line up with the barrel cut outs.  In the photo, the FSB is exactly where it needs to be.  I don't want to fill the gap between the rail and the FSB.  I want to fill the gap between the thicker portion of the rail and the FSB.  The little gap that is making you think the FSB is not seated all the way back, is the very gap that I want to fill.  It is the thickness of the handguard cap (less than 1mm).  

Just to better explain it, here is a picture with the FSB as far back as it will go.  The pins will not go in with the FSB in this position.  As this is an LMT, the barrel is not parkerized under the FSB.  And you can see that in this position, the unfinished area of the barrel is showing.  The FSB must be positioned as it is in the first photo.  Sorry for the confusion.  I didn't explain myself very well the first time.

Here is a photo of a Colt 6940 posted by Sabre223.  You can see that directly behind the FSB is a small shim.  This is what I am looking for.  

If it bothers you that much, why not just cut down your HG cap to fit in there?  Personally I wouldn't worry about it.

Cutting down the handguard cap is not a bad idea.  But, if someone sells a shim for this for cheap, I would just order one the next time I am placing an order for something anyways.  Thanks again for the suggestion, I just might go that route and cut one myself.

Gotcha-I understand now

If you don't want to cut your stock cap, I would wager that Adco probably carries the shim you are looking for.

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+1  I would imagine Lowe's or Home Depot would carry a 3/4 inch inner diameter washer or shim.  When I switched to a Larue rail, I also switched to an A.R.M.S. flip up front sight/gas block and it came with a little black shim to cover up that gap.

I know it's cosmetic, but it would bother me too.  And for a shim that shouldn't cost more than a quarter, why not fix it?

Will the whole HG cap fit in there under the rail, or does it HAVE to be cut down? I know it will fit under some rails, but I'm not sure about the laRue rail.

I'm asking because I have a LaRue 9.0 on order, and had planned (absent-mindedly) to use the whole HG cap....

I think Armalite offers spacers.

Nice.  Adco probably would have something.  Thanks for the tip.
I'll give them a call on Monday.