Why is a bar drink called a shot?

Its a nickname fromt he west. When cowboys entered taverns for a "dual" they ould set rules before hand. A main rule in these duals was that you could only use a single bullet. A single shot. Whoever was the winner was usually treated to a small drink of there favourite alcoholic drink. The traditional term of measuring how much went into the glass was how long the gun shot sounded for. This coincidentally was 45 ml. The length of time a gun shot sounds for is how long it takes to pour 45 ml of liquid. After a while this was forgotten and was served in pubs and taverns as a measurement.