elper,
I am interested in your question, not because I have an answer, but because I would like to retire and buy enough land to have my own pistol/rifle range, and have a similar question.
The answers you have received are, I am sure, correct, if you have flat land. Now, if your land had some sort of a natural backstop, such as a near vertical wall of sufficient height, then 5 acres might suffice. If the fields of fire were oriented correctly.
My problem is too find enough land with a natural backstop which could meet the most stringent safety considerations I can find. You have to make sure that any bullet you fire will NOT impact in populated land.
Frosty
MOLON LABE