AFter doing some research, I found out that the AR-15 pistol with its short barrel can in effect be used as an effective self-defense weapon.  I was reading on the HK-53 weapon, which uses the 223 round and also has a short barrel (a little over 8 inches).  Sounds like many SWAT guys are wanting the HK-53 instead of the MP5 due to stopping power; using soft nosed high velocity ammo for the rounds to fragment.  Check out this URL:  http://remtek.com/arms/hk/mil/53/53.htm

Just thought you guys may be interested.

I've been saying it for a while now. You CAN get 5.56 to fragment out of 7.5 inches if you use heavier ammo.

Check out the range report on my blog: www.anathema-incarnate.blogspot.com

And if you miss their face by a hair at least you burned their eyebrows off :)  Jk, but I'm seriously considering building an 11.5 bbl AR pistol for my girlfriends home defence weapon.  She is a small statured woman and even a youth 20 gauge is extremely awkward for her to hold.  I figure an AR pistol would do the trick, seeing as how it'd would be more controlable with the recoil, more pointable, less likely to overpentrate and injure anyone else, not to mention 20 or 30 rd mags give more chances to score a hit.  Im all but certain if it ever had to be used one shot is all it would take.  At the sub 20 foot range it'd be a bit like using a paintball gun, which she's done before, point and shoot.  Still more stability and easier to point on target than using an actual handgun.    

also it would scare the criminal shitless

Yeah, soft point ammo is widely known for it's fragmentation qualities