An officer involved in metro security said the incident was reported at about 3:00 pm at the Lal Quila metro station.
A CISF personnel detected a bullet-like object in a bag screened at the X-ray machine.
The bag belonged to a man identified as Sonu(24), a resident of Mathura in Uttar Pradesh. A live bullet round was recovered from the bag, the officer said.
The man was handed over to the police after he failed to furnish legal documents for possessing the ammunition, the official said.
Carrying arms and ammunition in the Delhi Metro is banned under the law of the land.