I have a Federal Firearms License.
At least I did until I let it lapse after buying all the guns I wanted. It's a Curio and Relics license. With it I could buy all manner of guns, up to and including 20mm anti aircraft guns from WWII. Hey, I could hunt airliners from my back yard! You'd be surprised what ten grand will buy.
Now, in my opinion, ain't nobody I know needs or should own a semi automatic gun of any kind. You may be an exception, you may live in Syria. I don't and don't know anybody who does.
Oh! But the AR-15 is America's gun, the most popular modern sporting rifle in the US. Sure it is 'cause you're too damn stupid to not be duped by the gun makers. Yeah, like suppressors that will save your hearing--hint, a $20 set of ear muffs work better than the $300 suppressor, before you spend another $300 to modify yer gun to accept it.
Since the mid '70s the gun industry has marketed to the home defense demography. Be afraid, be very afraid, the bad guys with guns are coming for you so you need a gun too!
Let's get down to brass tacks, shall we. If you want a gun to defend yourself an AR-15 or any hand gun ain't it. A hand gun is too hard to aim, particularly in times of stress. The FBI tells us that most shootouts take place at 20 feet and yet the kill rate for those shoot outs are almost too low to report. Hell, more kids are killed in their homes by the bullets that missed the miserable son-uv-a-bitch being shot at than actually hit the miserable son-uv-a-bitch being shot at.
Now for the AR-15, it is a very good close in weapon IF you are in command of the situation and you don't give a shit about collateral damage. If not it's just a long pistol with greater muzzle velocity that will penetrate interior walls and find all sorts of unintended targets. Shit, the best case scenario is you'll kill the refrigerator and yer flat screen TV. Meanwhile the intruder, who might be yer teenage kid coming home late or your wife making a sandwich just has the shit scared out of them and left a messy brown trail for you to clean up. Not to even consider what happens if your wife or kid survives the encounter--wouldn't want to be part of THAT conversation. Humm, can a gun be fitted anally? What does that holster look like?
So lets go back to the pistol. If you can hit a 6 inch target at 20 meters (66 feet) which is the international standard for short guns and small bore you get through the front door. Now make the room dark and use a pop up target and if you can hit a 6 inch target at 3 meters (10 feet) not knowing where the target will pop up good for you. Now do it when you're in a deep sleep and the adrenaline if flowing and you're half way scared as shit and thinking the worst possible scenario. Oops, sorry honey. Didn't know you wuz hungry. Sorry kid, you know you missed your curfew, right? You're in real trouble now young lady!
The other problem with most hand guns and all semi military style rifles (the manly black guns with all sorts of tactical add ons) is that secondary discharges are likely. No, almost assuredly predictable, because let's face it you're scared as shit and you're finger is inside the trigger guard and against the trigger and a gun just went off! The first round may have been controlled, the second not so much. Aw WTF it went in more or less the same direction as the first round. Close enough, right? Who needs a microwave anyway, it was next to the refrigerator that got killed when my first shot went from the foyer through the living room into the kitchen.
Take it from someone who is a qualified shooter, has had military training and has rationally thought about home defense. If you can't make a bad guy stop doing what he's doing with six shots you need to work on your skills. You don't need a fucking gun with more than 6 rounds.
If you are really concerned, the price of a quality gun will buy a monitored home security system. Shit, if you can't afford that a few bucks will buy solar powered security cameras with lights and cell phone connection to monitor your home when your're out.
There is no need or reason or constitutional justification for semi automatic firearms of any kind. They should be banned up to and including existing ownership. There are many options for home defense that don't include auto loading guns and interchangeable magazines that do not violate the 2nd amendment.
If you can't make a bad guy stop doing whatever he is doing with 6 shots, you need to work on your skills.
= new reply since forum marked as read
Woof!
THANK YOU!!! 
A funny-but-now-disgraced comedian once had a routine about how he figured learning "the karate yell" would be sufficient self-protection, because if he sounded fierce enough it would scare off would-be attackers as they would think he actually could take them apart.
Since I am small, female, and physically inept, it sounded like a plan to me. 
