Celia Bigelow with American Majority Action explains the benefit of an AR-15 for self-defense to Piers Morgan

MORGAN: Explain to me why you believe you need an AR-15 style assault rifle. 

CELIA BIGELOW, DIR. OF CAMPUS ACTION, AMERICAN MAJORITY ACTION: Well, I personally bought one for self defense. One, they're lightweight. They're quite accurate. I can shoot them much more accurately than a handgun or a shotgun.
And three, these -- I'm going to pull a David Gregory right here and I can hold up my 30-round magazine right here, because it's actually legal in Virginia. But I want a gun that can hold a lot of ammo because if I'm faced with an intruder or multiple intruders that come into my home, I want to make sure I have enough ammo to get the job done, especially if they're armed.
So I want -- they essentially serve as an insurance mechanism to make sure that I have enough rounds, that if multiple intruders come in and they're armed, I don't have to take the time to reload.

The interview provides an example of someone using an AR-15 for self-defense.  Here is another case that just occurred.

This week, two students at the Rochester Institute of Technology in upstate New York used an “assault rifle” that may soon be banned by law to frighten away armed burglars who had broken into their home.
Local station ABC 13 reports that the students, Christopher Boise and Raymond (no last name given) were awakened in the wee hours Tuesday morning to noises coming from the basement.
Boise got up to investigate and found two men, one with a gun pointed at him, at the bottom of the stairs in the lower level of his apartment.
Hearing Boise’s cry of alarm, Raymond instinctively went for his AR-15 rifle, which he kept legally for self-defense.
When the men came to Raymond’s door, they saw the barrel of his gun at the ready and fled immediately.
“Nothing was taken and no shots were fired,” the station reports.
Police officials confirmed that the gun was legal and Raymond had acted rightly in protecting his property. . . .