Sentence:  A “line” was a unit of measurement used in tsarist Russia and equal to about a tenth of an inch. The 3-<WSD>line</WSD> rifle, therefore, had a bore of three <WSD>lines</WSD>, or approximately .30 caliber. 
Meaning of the word marked with <WSD> tags:  A tsarist-era Russian unit of measure, approximately equal to one tenth of an English inch, used especially when measuring the calibre of firearms.