“Hand” is not metonymy whenever an actual hand is intended. Likewise, the direction from “fingers” to fingertips is justified by the physical and verbal context; the same word need not always carry that transfer.
A qarina may be a practical impossibility, the selectional meaning of a verb, the persons and events under discussion, or the overall communicative purpose. These different indicators direct the hearer away from a purely literal reading.
One should analyze actual usage, not assume that a familiar Arabic expression possesses one fixed figurative label under all circumstances.