Perhaps you should move to a less lethal option. Nail darts at low speeds are fully capable of killing a human, far more so than your typical pellet gun. Paintballs are easy to come by and cheap. You could cast slugs made of plaster or wax, they work quite well for close range target practice. Switch to a lower caliber and shoot airsoft pellets. Those are some ideas that your parents would be a bit more comfortable with. At any rate, be sure you are always safe and responsible with your airguns, treat them like they are a firearm. If you do so, I think your parents will rest easier with your hobby.
