A "slug," in the context of mass, is a unit of measurement primarily used in the United States customary and British imperial systems. It is not a widely used unit and is considered somewhat archaic. The slug is used to measure mass and is defined as the mass that accelerates at a rate of one foot per second squared (1 ft/s²) when a force of one pound-force (1 lbf) is applied to it. In other words, a slug is the mass that results in an acceleration of 1 ft/s² when a force of 1 lbf is applied to it.
Grain: The grain is a unit of mass primarily used for measuring the weight of small objects or quantities, especially in the context of bullets, ammunition, and some food products. One grain is approximately equal to 0.0648 grams.