Nobody should ever fall into the trap of having to justify owning a certain gun.
Throw the question back at the person who asked it. For instance:
"Why shouldn't I own a weapon like an AR-15? You seem to have an opinion. You tell me."
"Why would my rights disappear when it comes to firearms that look different from hunting rifles?"
Prov 26:4-5
4 Do not answer a fool according to his folly,
Or you will also be like him.
5 Answer a fool as his folly deserves,
That he not be wise in his own eyes.
Stating, don’t accept the premises of a fool,
but answer him as his foolishness deserves.