On a tip-off about poacher movement inside the park, personnel of Forest Battalion Force, India Reserve Battalion (IRB) and Assam Police launched the operation last evening, KNP sources told PTI.
The security personnel fired 22 rounds in the air to warn the poachers who fled from the area, they said.
During the search operation, the rifle was found wrapped in a gunny bag and hung on a tree near Lal Dapu area.
The poachers had spread across the world heritage site for locating rhinos and to drive them towards areas where no human movement was sighted, they said.