About the only "thing" that may help in those type tragic situations is your wits, circa a fast thinking convenience store clerk in Phoenix ~1993: The bad guy walked in the store with his gun out, the cashier starts unloading the till into a bag, but feigns glances to the left of the bad guy a couple of times then says "hi officer smith". The bad guy turns all the way around to (not) see the (phantom) cop and is promptly shot in the back of head by the clerk and dies.
As Edward White of Apollo 1 said the week before dying in the capsule, "Everything we do in life is a risk".
Hopefully most clerks receive training on how to deal with this, but I expect not.
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.50 BMG: 1 Shot 1 THRILL!