Disassembling a firearm is NOT a legal way to circumvent shipping laws. The receiver is legally a firearm and is subject to all the laws that affect shipping firearms even if it is shipped all by itself with no other parts whatsoever.
YES
HOWEVER...
Disassembling a firearm, shipping everything BUT the frame to your final destination (via good old cheap USPS Priority Mail, if you like), and transporting the FRAME ONLY (so much the better if it's a bare frame) in your locked luggage will go a lot smoother at the check-in counter than trying to ship a complete firearm.
The check-in counter agent and the inspectors see that it is a FRAME ONLY, incapable of being fired, and generally regard it as being innocuous; Meanwhile you have complied with ALL Federal laws, BATFE, POSTAL and FAR Part 121.
Counter agents are working stiffs just like us; Some may not even be aware that a frame is still considered a "firearm" by the Feds and will ask why you are declaring it. You just reply (with an air of confidence and authority) that, even though it is "just a part," it is still
technically considered a firearm under Federal law. You may find that some agents will surprise you and agree that it is silly to have to declare it.
It works, and everything goes soooooo much easier.
One caveat: Try to keep your bags within view for as long as possible while the TSA guys are rifling through it; Some of them have sticky fingers.