I think the correlation with advancements in civic awareness will be greater than for firearms.
After all, handy firepower has been a very sudden, very recent change. A crossbow, while it did exist, and could provide good one-shot firepower was NOT in anyone's possession (significantly at least), and the decrease in violence has been more monotonous, falling also during pre-firearm times.
Also, if we look at ganglands; I think it's safe to say they're well armed, and yet violence remains. It's too easy to classify criminals as a different class of people from law-abiding citizens, so something else is at work.
I call it culture.
You mean a culture of greed? You may have something. Our societies raise phalanx of children with little to loose and a promise of incredible wealth is just a few drug deals away. Sure you may have to shoot a few people and you may end up shot to death, but when you are raised with no perceived other opportunities, and see the "Good Guys" for what they really are, what is there to loose?