"Why do you need an semi-automatic rifle?"
"Why do you need a 30 round magazine?"
These kinds of questions are designed to put you on the defensive.
You do not have to defend your choice to own a bolt-action, pump or semi-auto - or full-auto if you want to jump through the NFA hoops.
You don't have to defend whether your handgun holds 6 rounds, or 8 or 16 or 19.
The answer to these questions is, "Because I can."
No, the answer is another question, directed at them..
Them: “Why do you need a 30 round magazine?”
You: “Why do you feel a need to control what I legally obtained and possess?”...
stand by for the brain overload