http://www.bbc.co.uk/news/world-us-canada-20851532

Is it effective? Better than some moron shooting themselves if nothing else...

Whats to stop the person with the gun walking in with their guns and holding up the store?

{NRA}Or the people with groceries on the way home who now don't have guns to defend themselves from potential gun-wielding grocery thieves?{/NRA}

One justification I've heard from a friend of a friend on Facebook was that as the US shares a border with Mexico, everyone needs to carry a firearm.
That mentality is understandable, perhaps, from someone in say, Texas. This chap was in Kentucky, however.

Mexico and the US have fought the odd war or two in the past ( ref: the Alamo) but I suspect Americans have killed far more Americans than the Mexicans ever have.

Americans have killed more americans in the civil war than every other war that they were involved in.