Check your laws.
There are three types of air rifles.
Springer, RSWs, has a twang when you shoot it.
Pressure pump, Benjamen Sheridan. Works your body to take multiple shots.
PSP, scuba tank. Career II 707. You will need to get a scuba tank. It will cost a good bit more.
If you want to kill something with an airgun, size matters much more than speed. I settled on 22 because I couldn't go larger without paying big bucks and I couldn't get them in all three flavors.
Supersonic rounds make a crack.
One day, I shall come back. Yes, I shall come back. Until then, there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs and prove to me that I am not mistaken in mine.