This is obviously a sequel to this post, this other post about how non-weapon equipment and its mechanics can influence a game, and this third, fourth and fifth post about distribution of tech amongst the general population.
I can't entirely avoid talking about this stuff again, but it's a different angle.
What kind of weapons are commonly available to civilians is a huge deal. There's a complex mixture of legality, opportunity and culture here, but I don't claim to understand that.
One factor is the typical discrepancy between a civilian and a ne'erdowell. If armed civilians tend to carry the same level of weaponry as a criminal, it's more likely civilians will tackle criminals. This also makes it harder to pick out a likely threat from a crowd, be they would-be assassins or the police you're trying to avoid; you can't simply scan to see who's armed and focus on them.