OK. Maybe the correct stat is more like 10%. If you were going to try and buy a weapon, and you had ulterior motives, where would you buy your gun? At a shop with a background check... or the gun show without one?

My point being... even if the number is 5%, this is where people who don't want to buy one in an alley will probably go. Wouldn't it be worth stopping even 2% of gun buyers if they didn't pass muster?