Last Oct I went to Knob Creek and bought a new, complete upper for my AR as a souvineer. I put it in my checked bag, and notified the person at the counter that I had gun "parts" in my bag, and NOT a complete firearm. She did a doubletake and I had to tell her again, but after that she seemed ok with it, and told me that she would pass the info along to the inspectors. I also showed her that it was wrapped in bubble wrap. It looks much scarier than it is, as the freefloat hg extends all the way to the end of the barrel, making it look like a supressor to the uneducated.
When I got my bag once back at LAX and checked it immedaitely, everything was fine. When I got home and was unpacking, I found a card form the TSA informing me that my bag had been opened and inspected for security purposes. It had the date and the inspectors badge number.