I don't recognize the caliber (5.3mm), and I don't know how you got it home, but You should be glad that the stock never made it to the U.S.
If you possessed all the components (i.e. the stock) to assemble this gun, you would be in violation of the National Firearms Act of 1934 by possessing what is defined as a short barreled rifle (SBR) that wasn't registered with the Treasury ATF...
A "rifle", to be legal, must be have at least a 16 inch barrel measured from the muzzle to the breechface, and the overall length must be at least 26.5 inches... fail to meet one of these requirements in a shoulder-stocked weapon, and you have just become a federal felon...IF you haven't pre-registered the weapon with the ATF prior to manufacture...
The same thing applies to creation of a pistol from what was originally manufactured as a rifle.
If I were you, I would have the barrel removed and discard it or give it to someone else. You can then have it barreled to .22 LR caliber with a legal length of at least 16 inches, and then have a custom stock fabricated for it. It should make a fine plinker.
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regards, -John S
"...We hold these truths to be self-evident that ALL men are created EQUAL and are endowed by their Creator with certain UNALIENABLE rights, and among these are life, LIBERTY, and the pursuit of happiness..."
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