The caliber wars are never ending, but once spin to the argument that in my opinion is not much of an argument anymore, is that bullet designs are becoming more and more effective.    When you read about the 9mm vs 45acp debates people argue about the size of the bullet, over penetration and stopping power.   The reality is that hunting bullets and defensive ammo has taken a step, maybe not a giant step, but a considerable step forward in increasing a calibers effectiveness.

I was watching a instructional video where the instructor showed bullet penetration differences between a 38 special, shotgun with bird shot, buckshot, a 9mm handgun and an AR15.    Most novice shooters would think the rifle was the biggest penetrator and the reality was it was not.    It now depends on the bullet design and not the caliber.   A fragmenting .223 bullet will transfer energy much easier to soft targets and violently fragment and not over penetrate.   Something as mild as a 38 special with a FMJ can pass through an urban home much easier.   Your AR15 may be a better option as a defensive weapon if you select the right ammo.   If you own a 20inch barrel variant you may want to consider swapping out your AR15 Upper Receiver for something a little shorter just so manuevering inside your  home is easier, but it hands down far better than a  handgun for most defensive purposes.