Grains are used as a measure of weight, as stated above. 1 Grain is 1/7,000 of a pound. When a box of ammunition lists the number of grains, it is in reference to the weight of the projectile. Normally, an increase in projectile weight will decrease velocity. Any attempt to increase weight AND velocity will usually increase pressure. As a result, there are standard guidelines regulated by SAAMI for each caliber that specify dimensions, pressure and velocity. Generally, any commercially loaded ammunition will fall within these guidelines. However, handloaders have been known to experiment with loads the go above the listed maximums for a given caliber; so be careful with reloads.