I wonder how he study would be if they just looked at those states which had Shall License for at least ten years. That way we know how things are after people had time to get their guns. In Illinois there were/are lots of restrictions to slow down the permits. I would say that few people that want concealed carry actually have those permits.
Then you have to wait until the criminals get mowed down a few times for the deterrent affect to take hold. I am not sure what the numbers would be. But watching Illinois and Wisconsin, I see that concealed care is generally a good thing. But intercity black neighborhoods have and had guns. And they are less safe now under both types of laws. In other words, black neighborhoods have guns regardless of laws. White neighborhoods get more pistols when the laws allow. But black neighborhoods seem to go after blacks. Whites seem to rarely suffer from violence, even when they are next to a black neighborhood. Theft is an issue but violence is far worse for blacks in or near a black neighborhood than whites.
John Lott says the greatest decreases in violent crime from increases in concealed carry permits happen in urban minority areas.