Boric's main action against silverfish is as a desiccant. This means that it kills them by absorbing all the moisture out of the insect's body. Without moisture, the silverfish cannot survive and quickly dies.

Boric acid's main advantage is as a desiccant. But it is also an effective poison. It can even kill humans but must be eaten in huge quantities - hence it is considered safe for home use.
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But if a silverfish eats boric acid then it will quickly die. This is the basis of some types of silverfish traps - they are made entirely of cardboard that is saturated in the acid. Silverfish see the cardboard as food (carbohydrates are the mainstay of their diet) and they eat it, oblivious to the fact that it will kill them.

If gun safes are necessary to keep your gun safe, then it is equally important to keep it dry and clean. A closed and humid environment attracts microbes and fungi and it is essential to keep them in check. There is no safe that is free from this problem. Hence it is we who should be cautious of this problem.
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