My problem with this argument is that it's fundamentally flawed. Of course they don't go to get their guns "legally" because it is illegal for them to buy them at all. If by legally you mean from a legitimate retailer, then it seems that the background check is doing exactly what it was designed to do.
I agree, they background check itself does not inherently prevent nefarious actors from getting a gun. The real beauty of a background check is that it forces felons intent on getting guns to commit a non-violent crime before they even use the gun to commit a violent crime. As controversial as it was, NYC's stop and frisk policy was incredibly effective at pulling illegally obtained guns from criminals and gang-bangers and taking these people off the streets.
It sounds like your issue stems from the fact that you have delays. Have you pursued a VAF? It sounds like there's some things that needs to get straightened out. As I stated, my background beck is usually complete and passed before I get to my dealer to pick up the gun (since nothing seems to be in stock lately). I hardly consider this to be an infringement. I honestly don't even notice.
Information and frequently asked questions about the NICS appeal process and the Voluntary Appeal File (VAF).
www.fbi.gov
As I stated, there are a lot of things I consider to be a legitimate infringement on my 2A rights, including permits to purchase handguns and NJ's effective ban on carrying a weapon. Most other laws, like magazine capacity and bans on collapsible stocks are pointless laws that turn ordinary people into criminals with no impact on safety, but aren't an infringement per se. I firmly, yet respectfully, disagree with your position that a background check is an infringement on your 2A rights, and hope that you recognize the value in background checks that do not support some sort of database of firearms owners.