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The compact “carry” handgun is a very short range weapon.
Only a complete fool would attempt a long range shot to take out a mass shooter. It would risk an innocent death, and a manslaughter conviction for you.
If you are not close enough for a high percentage shot, the best policy would be lay low behind cover for a better opportunity to develop.
Heroics can be very foolish.
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I can make repeated hits into a six inch circle at 150 feet using S&W Chiefs Special that I have owned since 1971. At 200 feet the circle needs to be 8 inches for consistent hits. That little gun and I are long time friends.