In order import a firearm (or the frame of a firearm) into the US, you must be licensed by the US Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE, also called BATF, and ATF- sort our tax man) as an importer of firearms. The US State Department, as part of an international treaty, also has limitations/restrictions on the export of certain weapons related items (such as a rifle telescope that is usable as a military scope) and may require import PERMITS. Some items cannot be imported into the US, due to it's country of origin, and trade restrictions/ trade sanctions.
This is legally a VERY complex playing field- it involves multiple laws from 2 nations-and it is rather akin to playing football on a field that has been liberally sown with land mines. With about the same results.
Simply shipping a bolt one week, a barrel another, a stock the 3rd, and the action a 4th will accomplish nothing except probably getting your body in prison- all at the same time, however.
If you wish to pursue this, you will need bonafide legal counsel, which, I repeat- this is NOT.