The bore of a gun can be examined by:-: (A) Broach cutter, (B) Helixometer, (C) Comparison microscope, (D) Simple microscope
Answer is B. Helixometer: An instrument for inspecting the interior of a gun barrel and for measuring the rate of twist of the rifled bore of a firearm. Comparison microscope is used to match the marks in test bullet & crime bullet Broach cutter is used to produce riflings in a gun.