Grain is a measurement of the weight of the bullet (the amount of powder is also measured in grain). Various bullet styles are available, each having a standard size (weight). For example, if you want a hollow point, it will have a certain weight available. If you want a round nose full metal jacket bullet, it will usually be heavier than a hollow point. Generally, a heavier bullet will travel slower than a lighter bullet if the amoiunt of powder is the same. But different bullet weights often come loaded with different powder charges, so you’d have to check the specified bullet speed if you are interested in a certain speed. From here