Because slugs have much more bearing surface, and therefore friction, especially as weights go up, they fly slower for the same settings. While you can tune for maximum power, you won't get an idea about actual performance until you push the actual projectile. The other thing I find is slugs like faster speeds (I suspect because they are more in need of the spin speeds that come with higher velocities) and can have the opposite reaction relative to speed than pellets. For example a slug often becomes more accurate going from 800 fps to 1000 fps, while a pellet will often become less accurate. Because of all of this, I find many airguns prefer a considerably lighter slug than pellet. The pellets will often outperform the slugs, for power at close ranges (15-30 yds), but the slugs win as distances increase and they lose less velocity. They also buck the wind better which shows up again, at further distances.