Officials said the incident happened at a battalion camp of the Indo-Tibetan Border Police force in Karera in the district when the troops were undertaking a firing exercise.
A jawan lost his eyesight in the blast and has been referred to Delhi for treatment while a sub-inspector of the force accompanying him was injured.
They said preliminary reports found that during the firing practise of a Medium Machine Gun (MMG), a bullet reportedly got stuck in the barrel and the one behind burst inside hitting the two men.