Oh, do you think that isn't happening?
https://en.wikipedia.org/wiki/Gun_show_loophole
Now, what if I told you there is video evidence of it happening, even from "dealers", because no private person, would have over 500+ AR-15s to sell in one fucking weekend. I can't seem to find the video anymore, but it was literally a sting done by NBC a few years ago, where a guy went into a gun show, and came out with multiple handguns, and a rifle, without showing an ID, even though it is illegal to buy guns when you don't live in that state.
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Again, I am trying to find the video, but it literally happened.