There is no such thing as a light weight .308 AR. Some are lighter than others. But most will push 10 lbs. With a scope and magazine you are around 12-13 lbs...maybe more. There's only so much you can do in terms of designing a semi-auto rifle around a cartridge that will push 60,000 PSI.
DPMS makes a .308 Recon version that comes in a hair under 9 lbs. But that's about as light as you will find. Add 2 lbs for the magazine and another lb or two for the optics and you are pushing 13 lbs.
Consider a 6.8 SPC which has 70% of the energy of a .308 and can be had in a very lightweight AR size rifle. Ammo will be cheaper than comparable .308 Ammo too.
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Last edited by Big Bird; 12-04-2012 at 14:59..