"The killer in Las Vegas apparently used the legal bump stock devices on legal rifles, . . . exposing what some lawmakers said looked like a loophole in gun laws."
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the device is not necessary to simulate full auto-fire. ban it if you will. the trigger spot isn't that difficult to locate for experienced shooters with a tuned trigger pull and moderately firm recoil control.
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however, anyone dedicated to killing people will not be deterred by firearm laws no matter how restrictive they may be. 
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rather than focusing solely on the devices used to kill, the public would be better served if those who are anti-gun worked instead to unravel what motivates a person to become a serial or mass murderer.
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honolulu probably still has not studied why the pali sniper or the kakaako parking lot sniper began their dangerous shootings. it's a difficult endeavor. it's easier to ban something, feel good and call it  solved.