Aftermarket modification kits are available for firearms such as the AR-15 that would upgrade a legal semi-automatic rifle into either a plain, fully automatic rifle or into selective fire assault rifles. They are available online for around $150.00-$1000.00 without any background checks and are shipped straight to your door. Once they are fitted onto a previously legal firearm, that firearm then is in violation of the National Firearms Act of 1934, the Gun Control Act of 1968, and finally the Hughes Amendment in 1986.
This loophole has drastically expanded with the proliferation of web-based merchants with a far larger audience via Google. Any search for slide trigger, full auto conversion kit for AR-15, sear trigger or lightning link will bring up purchasing options.