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 claimed that his brother worked as a constable with the Haryana Police and the bullet might be his.
Carrying arms and ammunition in the Delhi Metro is banned under the law.