An interesting read on the physics of pellet weight and velocity from the Umarex Blog.
In any given caliber or type of projectile launching device, there are three basic variables, the shape of the projectile, the mass of the projectile, and the force of the launch. Changing any of these variables results in different performance based upon the variable altered.
Common pellet shapes have various performance characteristics. If you are going for speed and penetration, use a pointed pellet. Looking for accuracy in target shooting? The flat tipped “wad-cutter” style pellet makes scoring targets easy, as well as generating the accuracy at 10 meters to win matches. Looking for a good compromise in range, accuracy, and power? Domed pellets tend to fly good and transfer energy to a target well. There are some novelty pellet shapes on the market, but those listed constitute the basic and most popular designs.
For pellet weight, we will focus on .177 caliber for the example. .177 caliber pellets can vary from 4.5 grains to 16.2 grains. A 16.2-grain pellet is 360% heavier than a 4.5-grain pellet! Remember our example using .223 Remington? A simple change of 15% made a huge difference in power output and a significant difference in velocity. To put things into some perspective with a comparison to .223 Remington, 40-grain bullets are the lightest while the heaviest .223 bullet is around 85 grains. This calculates to about a 212% difference between the two.