When one of my 22s quit firing reliably, it was because the firing pin had been peened down so far that it hit a frame stop as the pin contacted the rim of the case, resulting in an inadequate dent for detonating the primer charge. This happened because it got dry-fired a lot by others.
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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)