If I was a criminal I would not shoot someone, then throw the gun on the ground and run away .... guns cost money .... unless I was paid to do the shooting and then leave the gun behind .... something like a paid hit man.
Not just "hit men."
A lot of gang members who kill a lot of people will drop the guns at the crime scene, to ensure that if they're ever caught, they won't be tied to the crime like they would if they had the gun. Besides, the drug cartels don't have any trouble getting replacement guns, and on the world market, full auto AKs are a lot cheaper than even an SKS in the States would be. And the cartels can get them by the container load.
Mark