Sentence:  The FBI maintains a database, the General Rifling Characteristics (GRC) file, which is organized by caliber, number of <WSD>lands</WSD> and grooves, direction of twist, and width of <WSD>lands</WSD> and grooves, to help an examiner figure out the origin of a recovered bullet. 
Meaning of the word marked with <WSD> tags:  The space between the rifling grooves in a gun.