Mundaka-2-2-4-praṇavo dhanuḥ

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Com.—What the bow and the rest above referred to are, is explained. The Pranava, i.e., the syllable “Om” is the bow; as the bow is the cause of the arrow entering into the mark, so the syllable “Om” is the cause of the Atman entering into the Brahman; for it is only when purified, by the repetition of Pranava, that the Atman supported by it becomes fixed in the Brahman without obstruction, as the arrow by the force of the bow is fixed in the mark. Therefore the Pranava is like a bow. The arrow is the Paramatman itself conditioned as the Atman having entered the body here, as the sun enters the water, as the witness of all states of consciousness. That, like an arrow, is discharged towards itself—the immortal Brahman. Therefore the Brahman is said to be its mark, because it is seen to be the Atman itself by those who fix their mind upon it as on a mark. This being so, the Brahman which is the mark should be hit by one who is self-collected, i. e., who is free from the excitement caused by a thirst to get at external objects, who is disgusted with everything, who has conquered his senses and whose mind is concentrated. When that is hit, the Atman becomes like the arrow, one with the mark, i.e., the Brahman. Just as the success of the arrow is its becoming one with the mark, so the fruit here achieved is the Atman becoming one with the immortal Brahman by the dispelling of the notion that the body, etc., is the Atman.

Links
Various resources on the Mundakopanishad
Upanishad Series – Mundakopanishad Commentary by Swami Chinmayananda