If you shoot a charging bear at 20 yards in Alaska, it is very likely you will be fined heavily. To be considered defenseable in life or property, the bear better have powder burns on it when you kill it. I'm only half joking. As described above, it is very difficult to tell when or if a bear is going to stop its charge. The bear doesn't know.

Best is to plan on one shot, make it close and make it good. Unless you have a bear tag in your pocket...