District Detective Branch police said they detained Didar Alam, 30, and Afsar Mia, 30, at a checkpoint in the Shitalpur area of Sitakunda.
Police linked the detentions to their investigation into the June 13 killing of Maksudul Haque Chowdhury in Chattogram's Pahartali area.
They said investigators used CCTV footage, technical information and intelligence in identifying people of interest in the case.
Additional Superintendent of Police Md Russell said a foreign-made pistol, a magazine and two rounds of 7.65mm ammunition were recovered from beneath the ground beside a safety tank at a house in East Rauzan.
Police said the recovery followed information supplied by the two detainees.
Police alleged that the men were associates of a person accused in the murder case and had been involved in organised criminal activity, extortion and territorial disputes in Rauzan, Rangunia and nearby areas.
Police also said Didar Alam had 10 pending cases and Afsar Mia had six pending cases involving allegations including murder, weapons offences, extortion and criminal activity.
The supplied evidence does not state the detainees' legal response, whether they were remanded or charged after detention, or the outcome of any of the pending cases.