Well, FSB is an intelligence agency, and they wear guns or at least have a license to wear gun.
Though I cannot argue. I rely on my intuitive knowledge of Russian which is hard to use as a rational argument. This knowledge tells me that "силовые структуры"/"structures of force" are organizations that use force to solve problems. And I think that all of them wear guns. But I may be wrong with that last statement, and moreover language is changing, so the generally accepted meaning of the word can differ from what I had learnt in my childhood.
They are also a military branch and have military ranks, although they don't wear uniforms. The FSB acts like a combination of FBI, a border patrol and secret police.
They are also the group allowed to carry out an assassination on foreign soil (according to Russian law), although they are not allowed to authorize such a mission on their own.
Though I cannot argue. I rely on my intuitive knowledge of Russian which is hard to use as a rational argument. This knowledge tells me that "силовые структуры"/"structures of force" are organizations that use force to solve problems. And I think that all of them wear guns. But I may be wrong with that last statement, and moreover language is changing, so the generally accepted meaning of the word can differ from what I had learnt in my childhood.