Looking to add a carbon fiber hand-guard (rifle length) to my ongoing AR build. In particular I'm looking for an octagon shaped CF hand-guard, preferably without the pic rail on top. All I need to hang off the hand-guard is a bipod.

Suggestions?

I've never seen an octagon CF. Show us a picture if you find someone that's making them.

Clark Custom makes a nice light round CF HG. I'll be using one on a 6.8 hunting rig.

http://clarkcustomguns.com/parts-category/ar-15/

Looks like Lancer has them, but they are pricey:

http://www.lancer-systems.com/advanced-weapons-components-2/products/octagon-handguard/

Hadn't seen the Clark Custom version, albeit round. Cam across the Lancer after I posted here, but as you say - pricey.

Also came across this, again round, not octagon.

http://www.deltateamtactical.com/Carbon-fiber-Free-Float-Hand-Guard-12-Rifle-length-_p_4182.html

That would probably work fine. I bet it would be really light with all of the rails removed.

Before I would use a CF hand guard may I ask what barrel and stock you are using? Reason I ask is, with a 18" light contour barrel and a alum 12" handguard the Magpul UBR still comes out a little heavy in the butt on a 308 version. I'm looking to lighten up also but I can't really see how to do it without screwing up the balance really bad.

Stock - ACE Skeleton
Barrel - Christensen 16" CF wrapped

That may just work out real good.
I'm interested in hearing more about your little project. What's the weight looking to come out at? Minus optics and Ammo.

I'll check back in when it's all together. Upper and lower are currently out for Cerakoting.

Checking back in . . . after a short hiatus, filled with all to many projects.

Got the AR built and weighed. Tips the scale at 5lb 11oz with an empty mag, sans optics or sights. 2oz heavier with a Magpul CTR.

Anderson A4 upper, lower, and LPK sans trigger
RRA NM 2-stage trigger
Delta Tactical 12" CF hand-guard - no rails
Hogue grip
Ace Skeleton stock
Mid-length buffer and spring
Christensen 16" CF wrapped .223 Wylde barrel w/Miculek brake
Christensen provided low-profile gas block and tube

Mounted a Leupold VX-2, 4-12x40 AO on it and shot it last weekend. I am quite satisfied. Easily MOA with Fiocchi 223 50gr V-Max.

Here's one.

http://www.deltateamtactical.com/search.asp?keyword=carbon+fiber&search=GO

Actually, I purchased one of those (16.5" Octagon) and used it on another build. Since they didn't offer the 12" in same style, I went with the round version.