I find that S&W weight to be a little on the low side. I have a BCM 14.5" midlength with 11.0 Troy Extreme and it's 6 lbs 2 oz. I can't imagine that the longer barrel and tube doesn't weigh more.
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GotM4 does, you should PM him and ask him to build you one.
the cav lowers are pretty neat, i had one with a lightweight CMMG upper on it. i never weighed the gun, but it was probably the lightest AR15 i've ever handled. ~6 lbs probably?
that said if this rifle is for a woman/child an adjustable stock may help more than having the absolute lightest lower.
I have owned 2 of the Bushmaster carbon guns. The first I bought used. It would not feed, so I sent it back to BM and they fixed it for free. Then it fed but shot huge groups (XM193 ammo), and my confidence in it was already low. The other one I got new. It fed and ran fine, but also shot huge groups. So I ditched both of them. A shooting range near here that rents guns including MG to shooters had one of the BM carbon fibers, and customers shot it until the frame cracked.
Carbon fiber seems like a good idea, but my guess is it allows too much flex in the upper, resulting in poor accuracy, and the fiber frame needs an aluminum skeleton to stand up.
Thanks brother. Here's the hotlink to Project Featherweight.