First off, forget the HBWC backwards or any other handloaded ammo. You're getting into bad juju from both the forensic and liability standpoints. Accuracy can also be problematic.

Now about headshots. I've had the opportunity to observe the aftermath of a couple with handguns. The head is a lousy target. It's heavily protected and extremely mobile. Besides which, in general, if you're shooting at someone's head, as noted above, there's going to be a lot of other heads in close proximity. It may be the only target, but may not be your best option to resolve the issue. There are other ways to defeat body armor, but I'm not going into them on an open forum.