Matthew Allen, writing at his blog Straightforward in a Crooked World, offers some advice when dealing with riots or spree shootings:
When the wolf comes to kill don't worry about the sheep, don't worry what kind of dog you may be or may not be. Don't worry about getting killed, don't worry about the aftermath, don't worry...about anything.
Make a decision to stop the wolf and then go stop him.
Stop him where he stands by whatever means necessary and don't do it with kindness, don't do it with anger, do it with sincere an solid intent that he will never stand again.
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It's not caliber, capacity, polymer nor steel that overcomes the evil of this world but rather bravery.
Be brave. Be Brave. Be Brave.