Nope. Real-deal, semiautomatic weapon, according to the Wall Street Journal. Executive producer Jill Danton explained how it went down:
A camera operator "yelled, 'Look, that looks like a gun.' Oh, look, a fake gun' and he tosses it over the concrete ... "
An off-duty cop working on-set did a double take and ID'd the gun as a real pistol
The WSJ states that Culver City police will test the gun to see if it can be matched to unsolved crimes.
Good work, Law & Order. By the way, the episode they were taping is called "Ballona Creek" and aired Wednesday night.