The country-made weapon was found on the basis of the information provided by Salim, the main accused, during interrogation, Superintendent of Police, Piyush Srivastava said.
Two arms licences issued in Salim's name have been suspended following a directive by District Magistrate RP Singh.
"There were two licences - a revolver and DBBL gun - in Salim's name. It is also being probed if they had been used during the January 26 incident," Mr Singh said.
Chandan Gupta was killed in communal violence that broke out in Kasganj on the Republic Day. He died of gunshot wounds. The killing led to a spiral of violence in the western UP town. At least three shops, two buses and a car were torched in the violence.
Following the violence, the administration cracked the whip on a WhatsApp group operating in Kasganj and arrested the administrator for posting objectionable and communally inciting messages.
On January 31, the police claimed to have held Saleem, who they called the prime accused in the Chandan Gupta murder case. On February 3, one more person was arrested in connection with the killing.