Mangaldoi wildlife division's conservator of forest Sushil Kumar Daila said that the rifle might have been left behind by the gang of poachers during encounter on February 5.

"We suspect that the M16 belonged to the slain poachers," Daila added.

Generally poachers carry .303 rifle for killing rhinos. Wildlife crime experts said recovery of M16 rifle is a clear indication that poachers are now using sophisticated firearms. They added that in several poaching incidents at Kaziranga national park, on the southern bank of Brahmaputra, there were evidences of poachers using sophisticated firearms for killing rhinos. Many Northeast insurgent groups also uses M16 rifles. The firearms were used in 1960s by US military in jungle warfare during Vietnam war.

The Orang park officials on February 5 engaged a gang of five poachers in gun-battle when they sneaked into the protected area to kill rhino. Two poachers were gunned down, while three managed to escape. The poachers however, failed to kill rhino following park official's prompt reaction soon after the poachers gang entered the park under the cover of thick fog.

Orang, which was recently declared as tiger reserve, has 100-odd rhinos, and is also known as mini Kaziranga. The park located on the northern bank of Brahmaputra recorded zero rhino poaching last year, but the park was under constant pressure from poachers. Also last year, the park authorities were also successful in getting conviction of two poachers.

In 2014, the park lost two rhinos to poachers, while several poaching attempts were foiled.