A lot of that has to do with what state you live in. Here most go from person to person private transaction and its perfectly legal. I guess someplace of course the gun had to be bought new but Lord only knows how long ago or where. So I would have to say based on a lot of states being the same as Ky in that no FFL or background check is required for a private sale its higher than 40 percent.
That wouldnt be counting gang bangers etc that get their guns under the radar regardless.