Motihari: Acting on a tip-off, East Champaran police on Sunday busted an illegal cartridge manufacturing unit operating in Ojha Tola under the Chakia police station area and recovered over 500 live and semi-finished cartridges, along with weapon-making machinery.Four persons were arrested in connection with the racket and are being interrogated.East Champaran SP Swarn Prabhat said this was the first seizure of a cartridge manufacturing unit in the region. Preliminary investigation suggests that the clandestine factory had been supplying cartridges for various firearms to criminals in Bihar and Nepal for a long time.The operation was led by the SP himself. Among those arrested is Srikant Singh, alleged kingpin of the racket.Police recovered around 500 cartridges, pistol magazines, semi-finished weapons, gunpowder, empty cartridge casings, and a large quantity of explosive and chemical materials used in manufacturing ammunition.