200 fps with a 5.1 grain BB translates into .5 ft-lbs of muzzle energy. English sparrows and mice require 2 ft-lbs of impact energy for clean kills. So this gun doesn’t have enough energy at the muzzle to kill even the smallest small game animals, vermin, or nuisance birds. Additionally Marksman air-pistols have a reputation for being inaccurate, even among cheap BB guns… 4-5 inch groups at 10 feet are the norm, not the exception. So its doubtful you could hit a mouse or English sparrow. That said, if you did hit one, you could conceivably injure it seriously by breaking bones or causing internal injuries.