On a rifle overall barrel length must be less than 16" for it to be a pistol, that is from chamber to tip. A pistol may not have a stock of any sort because it is a "hand gun" capable of being fired with one hand. People like these because you can still use the hand guard and do all the stuff you can to a full size ar15. Generally on these pistols, barrel lengths are shorter than 12", 10" being common. Pretty nice for CQB and with the right rounds pack a potent punch. You can go shorter for certain, but loss of velocity and tremendous muzzle flash are enough to stop most people. Yes you can get Noveske anti-flashs and stuff like this, but overall it's a big waste of money to go shorter unless you have a specific purpose. Now, as cool as it seems don't let anyone catch you swapping a rifle lower (with stock) with a pistol upper. ATF and fed boys pay a lot closer attention now since the Obama scare and are ready and willing to do a lot more about it then what they used to...
Good choice on the OA, if I was going to buy one pre-assembled I would have gone this route.