My mental perception of the AR-15 is "chambered in EVERYTHING". From .50 BMG to .22LR to crossbow bolts. I wouldn't be surprised if you could get a slingshot upper.
"Got a knew AR upper the other day."
"Oh yeah, what's it shoot?"
"Rocks."

Anyway, here's this thing. No adjustable stock (I think it would be charitable to even call that foam tube a stock) and the barrel's only 7.25in, but it's 660$ and Bushmaster has a pretty good reputation. (right?)

Doesn't have a lot of range and it shoots 9mm. You might as well get a handgun for carry conceal and for home defense, you might just get a full size handgun.