Gun Ammo Grain Differences?

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

Another thing to note is that a heavier bullet, with the same speed as a lighter bullet have greater muzzle energy. Which in another words means it’s more dangerous!

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.