Acting on a tip off, the STF team raided Pramod Rai's house and arrested him, Deputy Superintendent of Police (DSP), STF, Gopal Paswan said.
The weapon had changed many hands before it came to Rai last year, he said.
Rai, an accused in the firing outside the residence of RJD MP Umashankar Singh at Chapra last year, was booked under the Arms Act, Paswan said.