Police unearthed a firearm-manufacturing unit at Rabindranagar in the port area on Tuesday afternoon and seized more than 100 guns.
Three men - two from Munger in Bihar and a resident of the port area - have been arrested.
"The three were running the unit. The duo from Munger, Md Aslam and Sheikh Ehtamas, were hired because of their expertise in manufacturing firearms," a CID officer said. The local hand has been identified as Aftab Hussain.
Munger is infamous for producing illegal firearms.
Police seized 95 pipeguns, four double-barrel guns, two 9mm pistols and 57 rounds of ammunition from the factory, which was set up in a secluded pocket in Rabindranagar's Bhandaripara. Seven cell phones and Rs 10,000 were seized from the arrested trio.
The police are yet to track down the buyers of the improvised firearms.
"We had a specific tip-off about the factory and shared it with our counterparts in South 24-Parganas. We are looking for more people involved in the racket," the CID officer said.
"Going by the number of firearms seized, it seems the factory had bagged a good contract," another officer said.
The police suspect the factory was set up six months ago. Last year, the CID had unearthed an illegal arms factory in Kalyani.
Officers of Baguiati police station arrested Manoj Yadav from Banka, Bihar, on Monday night for allegedly stealing valuables from his employer's house off VIP Road on August 6. Gold biscuits, a wristwatch and Rs 5 lakh were seized.