After lurking for awhile and checking the various forums, I notice a lack of substantive discussion about carbon fiber receivers (upper or lower).  I am in the planning stages of my first ground-up build and am curious.

What is the thinking on these items in terms of durability, reliability, accuracy, weight savings, etc.  Are they simply a new "gizmo" or is there some merit to their existence?

I have a set of the.....(cough, ahem, cough) Hesse receivers. SO FAR they haven't broke! That being said..............................................they will.

There is NOTHING lighter, also in AR-15 receivers, there is nothing WEAKER!

The problem is they use chopped carbon fiber, it's basically a fragile plastic. If some one were to manufacture a carbon fiber long strand mat fiber receiver it would be damn near indestructible, it would also be outrageously expensive.

They are nice for a range queen toy, not something to count on when it all goes wrong!

PursuitSS

Be careful.  I've seen the Bushmaster "Carbon 15" in person, and it's not the same "carbon fiber" that you'll seen in bicycle frames, helmets, automobile hoods, and free floating tubes.  It's just a fancy name for their type of polymer.

With that said, I don't see the point, unless you're limp-wristed and building an AR pistol.

OK... Since reliability is BIG on my list of reliable traits in a weapon, I should probably stay away from carbon fiber receivers, correct?

However, I notice with interest that cav arms makes a polymer lower.... what thoughts on that and do any other makers offer a similar item?

Go with metal lowers

Quoted: However, I notice with interest that cav arms makes a polymer lower.... what thoughts on that and do any other makers offer a similar item?

CavArms lowers are made from the same type of material as the Glock frame. The main complaint on ARFcom is that they have a fixed A1 length stock so they're not so readily adaptable to gear whoring or having crap magnets tied to them to be dragged through the inner bowels of the Equipment Exchange. They're made to shoot. They're simple, light, and durable. CavArms has a pix where they had parked a 3/4 ton truck with a lower under each wheel. The truck was pulled forward, an upper was placed on each lower, and all 4 fired.

I only have 5 of them and a token aluminum lowered AR just to say I have one. When I get the AK herd thinned out some more, may get another CavArms lowered AR for the empty slots. As far as non metal lowers, you've got the CavArms polymer, Bushmaster's Carbon 15, and the Hexxe lower. To be fair, I've not seen a report of a Hesse lower failing. You can probably guess that I'm going to recommend the CavArms lower.

wganz

Forged alloy uppers for me.  If you want a lightweight lower, go with Cav. Arms.--they've been around long enough and in large enough quantity to prove themselves to my satisfaction.  FWIW, I've never had any problems with mine.  Fore serious use, I'd pick a forges alloy upper and lower, but YMMV.

I own a bushmaster carbon 15 and like it. The only issue I have
with mine after 2000 rounds through it is the barrel seems to get hot
quicker than the aluminum receivers ive shot. But its way lighter than
any of my freinds guns.

AR aluminum receivers are incredibly light.  The weight saved by going carbon fiber is minimal.  You save the most weight in the barrel, rail system, and stock choices.  Your accessories can save as much weight.  A heavy or light VFG, light, and optic set up can add or drop up to a pound in weight depending on choices.