In other words, some guns have excellent safety mechanisms to prevent accidental discharge (for instance, when dropped), but there are also many that don't.
Technical/informative prose, probably from a firearms article. つまり ('in other words, that is') opens a clarification. 暴発 ('accidental discharge') is the precise firearms term. 落とされた is the passive past form of 落とす, used attributively in a relative clause. ~ために ('in order to prevent') is the purposive. そうでない ('not (of) that kind') is a contracted pointer to the opposite category. The sentence shows how Japanese can structure long technical sentences with clear logical nesting.