03 Marine here. Take this for what it's worth, which admittedly is what most internet posts are worth.
But, I think most folks seriously underestimate the advantage a rifle has over any side arm. At 30 yards, even a 10/22 has an advantage over a pistol. A rifle round will pass through most of what is on the shelves and still be lethal. A 9mm hollow point will likely not.
Your best bet, and even this is a long shot, is to try and ambush the shooter at very close range. Hide in a clothes rack, wait for shooter to close or pass, then engage. If you don't neutralize the target with a single magazine, your chances for success plummet. If the target can get his muzzle swung around to engage you, your chances for success plummet. Even vs. a noob with a POS AK.
I think the biggest challenge would be to maintain control over yourself. Your adrenaline will be raging, your heart pounding and palms sweating. You may suffer tunnel vision and the report from his rifle will be deafening if he is close. Add to that the screaming and yelling, people scrambling, running and freaking out.
Again. Take the above for what it is worth. But I think I am going to start carrying more often with a round that starts with a 4. Shit like this is the new reality in America. And while I like my PM9 and Sig 365 and 320, I will feel just a little more comfortable with a full sized 1911 and three spare mags in a shoulder rig.
I worry about copy cats. If another white kid decides to reduce the immigrant population...How long before Latin gangs decides to retaliate ? And they are not going to do it in downtown brown town. They are going to roll up to suburbia. And they are nationwide. If they pull it off with any type of coordination, the ringing you hear in the back ground will be the alarm on the go time clocks. Tin Foil hat? Maybe. Spare box of 45 in the truck? Definitely.