While I would agree that the #s of AR-15s in society probably doesn't effect crime rates too much, I do think that a generally well armed public does help reduce crime. I'm sure better policing and keeping the baddies incarcerated also helps.
There seems to be enough correlations between the crime rates of localities and gun ownership to reasonably believe it ameliorate crime. Studies by Gary Kleck of the University of Florida, Professors Lott & Mustard of the University of Chicago, and others, all seem to bear this out.
Now, sure, other things matter. Take New York City. Gun ownership there is extremely regulated since atleast the Sullivan Laws, and the city has suffered from inordinately high murder and violent crime rates in the 1950s up through several decades until Rudy Giuliani took over. But it wasn't through making guns easier to own that reduced the crime, it was better policing.