This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Ballistics, Subject name: Forensic Medicine
The bore of a gun can be examined by:- 
Select and explain. 
A. Broach cutter, B. Helixometer, C. Comparison microscope, D. Simple microscope
B: Helixometer
Answer: B: Helixometer 
Explanation: 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.