No one wants a cop killed, especially his department. So the New York Police Department, which has a Patrol Guide big enough to choke a carriage horse, dealing with everything from the tilt of a cop's cap to his nose hair, has no rules as to when a cop can draw his gun.

There are some New York City police officers who can count on one hand the number of times they have drawn a gun, even over decades on the force.

Then there are the officers who patrol the city's 334 public housing complexes. There are about 2,350 uniformed officers in the department's Housing Bureau, about 1,825 of whom are rank-and-file police officers.

To some of them, drawing their guns, even with no present threat, is routine, a practice borne of habit or some internal gauge of an encounter that might go bad. And their bosses, unlike some police commanders around the country, permit it.

When probationary officer Peter Liang killed Akai Gurley, his gun was drawn for no better reason than he could, he was doing vertical patrol in a housing project and, it appears, he felt more comfortable with gun drawn.  Just in case. Because you never know. Because stairwells can be dangerous.  But mostly, because he could.