When I was instructing at FLETC we taught that to have "dazzel effect" a flashlight needed a minimum of 13,000 candlepower. No idea what that translates to in lumens. More is always better. While it was not a true "stun" it would "dazzle" and cause an auto response to squint or close the eyes, and look away even if only for a moment.
Is that the idea they refer to by "stun factor"?
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"Most men take the straight and narrow. A few take the road less traveled. I chose to cut through the woods." ~Unknown~