It always seemed to me that an arsenal, as used by the police or press, was just a collection of guns in the hands of someone that they had a problem with. If the police raided Alsatian's house, for instance, he would be pictured as having an arsenal, and thereby looked at as a threat for just having the guns. Some of the police doing the raiding would have as many, or more than Alsatian, but would be beyond reproach due to their position.
For an antigun police chief or editor, any guns are unacceptable, and it would take just a few to make an arsenal. Imagine if the poor guy had a few thousand rounds of ammo!