It's all over the net.. Of course the 40% figure is an estimate, because pinning down the exact number is impossible.
Bit even if it's 1% that disproves the claim that every single gun sale requires a background check,..
As long as the sale is within your state, and to another person from the same state, it is perfectly legal. No background check is required.
Can't have it both ways. If you are a gun owner who says they are against criminals and the mentally ill having guns, then you can't be for the system we have now.