I managed to harvest a few pigeons with a 7.5 J air rifle at up to 15 m. Bullet placement is crucial. Accuracy sometimes depends on the bullets.
Pressurized air containers make the rifle quieter and can be refilled with an air pump (which is some workout and not too much fun).
There are air rifles powerful enough for deer. They are not legal around here.
I´d probably go for a slingshot. The ammo is more plentyful.