There really is not a gray area. ATF has put out several documents concerning AR lowers. When I checked last year, the following was legal to do. I don't know if anything has changed since then. Check their website to be sure.

If you start with a lower that has never been made into a firearm, you can make it into either a rifle or a pistol. If you make a rifle, it has to stay a rifle. You cannot convert it into a pistol in the future. If you first make it into a pistol, you can convert it back and forth to a rifle and back into a pistol as many times as you want. According to the ATF, the lower does not have to be marked "pistol" in order to be in the pistol mode. State laws may add some other regulations. Such as, California has many regulations about AR rifles that are not Federal regulations.