It was a Ruger AR-556.  Described here:
http://www.tactical-life.com/firearms/gun-review-ruger-ar-556/#ar-556-swmpam15-lead
as the "Battle Ready AR-556", "ready for any mission right out of the box", with "The mil-spec, six-position buffer tube holds an M4-style collapsible buttstock. "

So when the military imagery is used to sell guns, it's okay, huh?
The AR-556 is an AR-15, and you bloody well know that the AR-15 is always described as a "civilian semi-auto military M16".

Stop the BS.  The image of these guns as military is repeatedly hyped to their customers by manufacturers with a wink and a nod from NRA.  They sell these guns as toys you can use to pretend you are a soldier shooting bad guys.   Everything about these guns portrays them as military weapons.  And your egos feed on it.

Then, when some kook shoots down a church full of innocents, you pretend that the idea that these are military weapons is all made up by the nasty liberal media.