One thing to remember.... when the police get involved in your complaint, it becomes an official record. If the guy actually gets arrested, you've just gone from "someone should talk to him about gun safety" to:
he's gotta pay a lawyer (assuming he's got resources),
maybe pay to bond out,
spend work days preparing for and going to hearings,
perhaps have his firearm collection confiscated,
if his job is notified, and if he works at a place requiring a clean arrest background, he could lose his clearance or other credentials which means his job is at risk,
and on and on ...
Look at what happened to Zimmerman. He was cleared by the local cops, but Obama and Holder pressed for a trial. Even though he was acquitted in a trial that never should have happened, his life is basically ruined.
My point here is, calling the Cops should never be a routine decision. Not only are you setting the wheels in motion that may have unintended results, you are taking the police away from other, potentially more urgent, calls.
I take police reports very, very seriously, because the complainant is basically reporting they believe there is a law being broken. The Cops will investigate, and in this case, they decided to not do much. What if that had not been the case?
Maybe since you don't know your neighbors at all, other than observing their bad behaviors, you have no concerns about turning them over to the system. I ask you, what if this had been a close friend you observed. Would you have called the police?