Whats the rules on SBR'S and ar pistols?

I'm buying an AR "pistol" from a friend of mine. Its got a 10" upper on it. When I buy it whats the word on mods on it? I know if I put a stock on it its got to be registered as a SBR. But what are all the rules on it? Any?

I am not sure of "all the rules", but I do know that you cant put a stock on it. You can't put a vertical grip on the hand guard although a Magpul AGF seems to be ok. Can't put anything on the pistol buffer tube that would act as a stock that could be used pressed against the shoulder.