If you are looking for a unique, lightweight AR-15 style rifle, take a look at the Christensen Arms CA-15 carbines.  No doubt, the firearms are pricey, but they offer some features that are simply not available on other rifles.  If you are looking for a high end, semi-auto carbine that can run a variety of calibers from .204 Ruger to .300 AAC Blackout, you want to learn more about the Christensen Arms CA-15.