http://www.ar15.com/archive/topic.html?b=3&f=4&t=432545
Just depends on who makes it. Most are crappy.
Me, I agree with idea that there is nothing wrong with carbon fibre rifles IF engineered to be carbon fibre from the start. Using carbon fibre in place of aluminium and copying the original aluminium design without re-inforcement is a bad idea.
The AR seriesupper and lower were designed to be made from aluminum and unless redesigned for composites, not tinkered with.