Built on the AR platform, and piston operated.
Though with it's 30 round capacity I'm betting Bill Ruger wouldn't be happy.

http://www.ruger-firearms.com/SR556/

With an MSRP of $1995 they are out of their ever loving minds!!! Bill Ruger set out to design and make well made affordable firearms. I think the "affordable" part of Mr. Rugers vision has been permanently shelved. They really need to drop $1000 off the price.