A good high quality locking folder is plenty reliable these days as a stabbing weapon. However, if I got hold of the guy's shooting arm I'd slash his wrists hard and deep and he will be unable to pull on a trigger immediately. If he still struggles or the arms do not present a viable target, then a throat or face slash would be next. A stab wound to the thorax is not typically quickly incapacitating unless delivered in very specific areas and/or with great force.

BTW, don't stop with just one swipe or stab either. Once you've interrupted his attack, keep the pressure on full bore until he completely submits or is rendered visibly incapacitated.

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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)