A grain is a unit of mass, equal to 0.06479891 gram, in the avoirdupois, Apothecaries' and troy weight systems. An avoirdupois pound (i.e., the pound commonly abbreviated as lb in American usage) is 7000 grains, whereas a troy pound is 5760 grains.
Grains are still used as a measure for older drugs such as aspirin, though the milligram has almost completely replaced it. Grains are currently used in the USA to measure the mass of bullets and gunpowder, and scales for handloading measure in grains; bullets are generally measured in increments of 1 grain, gunpowder in increments of 0.1 grains.
A metric or pearl grain is equal to 50 milligrams or 1/4 of a carat as used in pearl and diamond nomenclature.
Originally, a grain was the weight of a grain seed from the middle of an ear of barley.
(Grains * 0.0001428571428571) = Pounds AV
(Grains * 0.0022857142857143) = Ounces AV, the 0.0023 factor HiWayMan posted.