Heckler & Koch MP5 that has a firing rate of 700 too 900 rounds per minute.
At first blush, it appeared that the script writer wanted to show that the police in New York City do not understand the difference between a semi-automatic weapon and a machine gun.
That's not unusual, a lot of people who know nothing of firearms hear fully-automatic or semi-automatic and they make no distinction. But the former is a machine gun and the latter is a single-shot firearm.
Upon further reflection, the script writer was saying something different, because later s/he intimates that the gun had been subjected to an illegal modification to convert it into a machine gun. So the writer was making a point about this illegal modification. Perhaps the justification for this narrative speed-bump was to make the viewing public more supportive of new gun laws.
But I'm more interested in the narrative speed-bump.
The audience tunes into a detective show on television to see all the things one expects of crime-fighting--searching for clues, drawing inferences from them, catching criminals, and bringing them to justice. Audience members can be equally entertained by this whether they are NRA members, ACLU members, or both. The audience has not tuned into a tract for any activist's cause.
Any speed-bump takes the audience out of the story--even those sympathetic to the political axe being ground. This is a tax on the storyteller. Part of what I call the Hollywood Stupid Tax.