Definition of precipitation for English Language Learners : water that falls to the ground as rain, snow, etc. : the process of separating a solid substance from a liquid
There is no standard unit of volume. In the US, a shot is 1.5 ounces and a small shot is 1 ounce of liquor for an alcoholic beverage such as whiskey. However, this varies internationally and there is no absolutely defined unit, and various sizes of glasses are used.
160 fluid ounces in a gallon, so there's 80 fluid ounces in a half gallon, standard shot glass is 2 ounces, so you'll have about 40 shots.