Does anyone in this post have a Carbon 15 rifle and if so how do they compare to standard AR15 builds?  Bushmaster claims their composite receivers are 40% stronger vs aluminum builds.  What are their major drawbacks?  

run a search on Carbon 15's and you'll get a load of info. Carbon 15's seem to have a lot of critics but I've never used one so I can't comment on them.

I have the new model#4,well actually I bought it for my wife my HBAR carbines are too heavy for her liking, it takes standard AR assemblies and parts.My aluminum upper fits the carbon lower and vice versa.I cant wait for the ban to go so I can add a telestock.So far it has 3000 rounds through it and no problems,alot smoother if you have to shoot dry than a regular AR would be.Also it came from bushmaster with a closed bottom carrier.