Spike,
Not really sure you could get much lighter than an AR platform in 7.62, 18" bbl with collapsible stock or folder on a piston gun. I have several both piston and direct impingement guns, I like the piston guns much better. I am on the fence between three rifles I would pick out of the safe if I needed to push back a fight. The afore mentioned FN/FAL, HK 91 or the M1A SOCOM. All are great rifles, all shoot extreamly well, but they are all heavy. I do however not consider the extra weight of a 7.62 battle rifle a deal breaker when it comes to a choice between 5,56 or 7.62, I will take the 308 every time. From what I have read on the Colt it seems to be a well built rifle, I just don't care for the bulk of the modular rifles such as the SCAR, ACR and the like, they feel like they were designed for a kid that grew up playing Nintendo. The battle proven three I mentioned rest at the front of my safe and I would bet my ass on them any day of the week.
Cheers,
Cody