Into the back garden is cool, provided as others have said:
a) BB doesn't leave your boundary
b) You can't be seen by anyone
As an aside - I remember having an air rifle when I was a kid and decided that the range in the garden wasn't long enough. So I set up plastic arms men on the drive and sniped their heads off from my bedroom window.
My Dad saw me doing it when he got home later that day and hit the roof. Biggest bollocking ever.
I explained I'd put in a backstop, and made sure the gates were shut, to keep it safe as possible.
He wondered what I was on about and said "no you idiot. I'm not worried about you shooting people, I'm worried about you getting shot!"
We lived backing onto an MOD property, a fact that hadn't really registered when I as in full on "I've had a cunning plan!" mode!