How gunmakers tweak rifles to get around assault weapon bans

California has tried three times to ban what lawmakers call assault weapons. Each time, gunmakers have made tiny tweaks to the offending rifles, and gun shops have continued selling guns that operate nearly identically to the banned models. WSJ explains how guns have evolved in the state, and why it can be difficult to make firearm restrictions effective. Photo: Michael Kofsky/Andria Chamberlin for The Wall Street Journal