Stripped lower purchased as a "short", or actual pistol now, and assembled as a pistol first (no buttstock) is an option for avoiding the tax stamp if you want a short AR. Keep in mind you still can't put a buttstock on with the short barrel without the appropriate form and tax.
If you own nothing of that type but do own an AR rifle, or a completed lower with buttstock installed, or a buttstock but no barrel 16" or longer, and purchase a short barreled upper you can be viewed as having intent to create an SBR (Short Barreled Rifle) by the ATF. That's bad if you haven't done the part Austin mentions.
"Clear as mud"
And the laws (a very flexible thing with the ATF) are changed or viewed differently, pretty often by the ATF. Simple route is to make a 16" or longer barreled AR first and bone up on the current state of affairs before trying for something outside the norm.