The "black guns" are their current target solely because they are viewed as the lowest hanging fruit, which is true. No one can claim they are worried about public safety and public health, then claim AR style rifles should be the top grab. In all your years as a LEO, how many times did you respond to a gun crime involving an AR vs how many times did you respond to a crime involving a handgun? They know a handgun ban is a much tougher hill to climb, so they start someplace else, but with the ultimate goal never out of sight.
Also, one of the most effective arguments they have on the AR front is that they are not able to be used for hunting, the "weapon of war" bullshit, to the degree we can mitigate that argument, the better off we are.