Actually no, most shooters do not own their guns legally. Only 18% of guns used in crimes were legally owned. Although your claim may be the case for mass shooters, a tiny minority of all perpetrators of gun homicide. https://www.washingtonpost.com/news/wonk/wp/2016/07/27/new-e...

Well, they may abide by gun laws in how they get the guns, but they sure aren't in how they use them...

(Though I suppose that's not "gun laws" that they're breaking, but rather laws against murder.)