Hi Nic,
This is not legal advice. My basic understanding is that you can use lethal force to protect yourself if you feel your life is in danger or you can use lethal force to protect someone else's life.
You cannot use lethal force to protect property. That means you cannot shoot someone if they are not threatening your life - so shooting someone in the back is a no-no - unless they are threatening someone else's life with their back turned toward you! Breaking into the car is also a no-no for shooting. I think there is precedent for believing your life is in danger if someone has broken into your house but you should check with a criminal or gun lawyer - Bruce Colodny is one local fellow.
I am in Los Angeles - may I ask what areas you are looking into investing in? I have my own favorite areas - mid city, east hollywood, etc. How about you?
Also - doesn't that RTF tear your hand up if you shoot a lot of rounds?? I held one at the shop and it was not comfortable to me!