Saying firearms aren't a deterrent is like saying atomic bombs aren't a deterrent. If you have the knowledge, the discretion, and the will to use them, they are the most effective deterrent available in their given gendre. Neither are 100% effective, but both are the best thing going.
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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)