N K Patel, who hails from a village near Bihar's capital Patna, was found carrying the firearm and 12 rounds by CISF personnel at the Dwarka Mor station when his bag was being scanned, a CISF spokesperson said.
He said the pistol was country-made.
The man has been handed over to the Delhi Police by the Central Industrial Security Force (CISF) team for further investigation, he said.
Carrying arms and ammunition in the Delhi Metro is banned under the law.