Let make an example about physic but in simpler therm that everybody will understand.
We got two cars, one SUV and one standart Sedan. I drive my sedan at 50kph in a wall and destroy that wall. Then I do it at 30 kph with the SUV and attain the same result. Why, because of the weight of the vehicle. While the SUV is slower, its still does as much damage. This is due to the two factors that influence kinetic energy which are speed and weight. In airsoft, shooting any kind of ammo with one gun will produce the same amount of energy since the spring transfert energy to the BB and not speed.