That is a 650 fps pellet rifle not just a bb gun
I have airsoft guns that have muzzle velocities in excess of 650 FPS, and I sure as heck would not be using them to euthanize anything just like I would not be using a 22LR to euthanize larger predators..
Fact is smaller projectiles generally produce less ballistic trauma making shot placement much more critical, if death vs maiming is the goal...