Fire (at, on, in, to) target
You can "fire at" or "fire on" them. To "fire in" would mean "inside" and "fire to" is just not grammatical. As to why some prepositions and not others, I suspect it's rather arbitrary.
Fire (at, on, in, to) target
You can "fire at" or "fire on" them. To "fire in" would mean "inside" and "fire to" is just not grammatical. As to why some prepositions and not others, I suspect it's rather arbitrary.