1. Many people use AR's for home defense. I'm going to venture you'd be amazed at how many people do.
2. I don't have any locks on my AR's, shotgun or handgun. Doesn't do me much good if it's locked up and I need it. I don't have kids, BTW.
3. I would argue it's exactly what the founders/framers had in mind. And armed society helps keep the government from runaway power.
All that's needed to control the population is to disarm them. The framers just finished defeating a tyrant and were pretty adamant about keeping that from happening again.
I'll fully concede that if I were to use a firearm to commit a mass murder, it would be an AR15 and not my deer rifle. For exactly the same reasons you said. It's lighter, more portable, magazine fed, reliable and if used by someone who knows what they are doing, extremely accurate, even in semi auto mode.
We are well beyond being able tbdefend from a tyrant. Your AR won't do squat for you.
Again...nobody is disarming you....