Gun Control & RKBA

And they wouldn't respond if I called and said someone was drunk either.
They would show up after the people were already dead. Since he wasn't breaking any laws until he open fired, the cops were prohibited from responding. Now the question of open carry, especially a rifle, in an urban environment would fall under "reasonable regulation" is better question. It seems to be a nonissue in Germany and Switzerland. Of course, you have to carry it unloaded and they use public transport more than we do.
Sometimes laws are not written simply because it would be a solution in search of a problem or simply out of sight out of mind. For example, when Wyoming liberalized CCW, part of the law was made that you could not carry concealed in a city or county council meeting. The law is silent on open carry even though it has always been legal, other than the reservation who have their own laws.
While she wasn't stoned.
http://ethicsalarms.com/2015/10/27/ethics-dunce-the-traverse-city-record-eagle/