Do you have any expirience with the Vltor monolithic uppers?
I've played with the carbine length Vltor upper. It was heavy as crap IMO. For the money and weight I really dont feel like your getting much, if anything. I much rather run a conventional upper and handgaurd. They way I run my guns I stand a pretty good chance of damaging stuff. With a conventional setup if I crack it, I can replace it. With an Vltor, that's not possible. I'm also a big fan of newer generation handgaurds that allow you to put rail sections where you want them, instead of having them built on. I really don't need that much rail real-estate and prefer the slimmed design of something like my Troy or the new Samson. I also like having handgaurds over the 12" rifle length that Vltor offers. The handgaurd on the 3-gun rifle above is 15". THis protects more of the barrel when I'm shooting from akward positions and off barricades.
Just my opinion