Current law requires those who are “engaged in the business” of selling firearms to get a federal firearms license — and to conduct background checks on buyers. Private sellers don’t need to conduct such checks. Gun control advocates have argued that administrative rules defining what that phrase means are too vague, allowing many private sellers who are actually selling guns as a quasi-business to do so without running checks on buyers. They argue that the Bureau of Alcohol, Tobacco and Firearms could tighten this up with a rule that narrows the definition of who is “engaged in the business.”

The Obama administration looked at this idea, officials tell me, studying whether “engaged in the business” could be defined with, say, a threshold number of guns sold — say, 50 or 100 per year. If this were done, those who identify as private sellers (and sell without background checks) but sell that many guns could no longer do so without getting a license and performing background checks.

But the idea quickly presented complications. One former administration official involved in these discussions tells me that some officials worried it would present new and unforeseen enforcement problems. One senior administration official says some worried internally that defining a commercial seller through a hard sales threshold — as opposed to, say, leaving it to the discretion of law enforcement to determine who is a commercial seller — could be subject to legal challenge and could end up sweeping in people selling guns who clearly were not engaged in it as a long term business. This could create untold logistical — not to say political — difficulties.

“It was very clear that it was way more complicated than the other stuff being looked at,” the senior administration official tells me.