Then why did the cop eject a shotshell from it and why was he cycling the bolt using a bolt handle on the SIDE of the weapon? All AR platforms use a rear charging handle. What was in that trunk was absolutely NOT a rifle and NOT AR-15 Bushmaster. I'll wager a paycheck on it.
http://www.youtube.com/watch?v=wLrxSgkqJQc
I believe it may have been a Saiga
http://ts2.mm.bing.net/th?id=H.4789155678850769&pid=1.7&w=248&h=54&c=7&rs=1
They make side charging handles for ARs.
That is absolutely not an AR rifle, but shotgun!! And a long gun at that. It looks closer to a standard Remmington 870 type hunting shotgun. That IS A SHOTGUN!!!! That was an expelled SHOTGUN round!!