Grain (often written as gr) refers to the weight of a bullet. The 115 gr 9mm bullet is the standard weight for the 9mm Luger, while the slightly heavier 124 gr bullet is the military style 9mm NATO. If all other factors are the same, the lighter 115 gr 9mm bullet will have a faster muzzle velocity, while the 124 gr bullet will have more muzzle energy.