Maybe this can help. I did a quick search and the second post found this on the BATFA.
Originally Posted by 19+15+1
I've seen lots of references to this law, but there seems to be some disagreement as to what exactly it means. I'm hoping someone can explain the law in plain English. I'm about to buy an M4 and would like to customize it but I'm concerned that I might do something illegal.
This is from another forum
"Technically, it prohibits the domestic assembly or reassembly of an imported firearm into a configuration that is unimportable. Once the Bush Import Ban took effect, people quickly discovered the legal loophole that allowed the importation of neutered guns and their restorative parts separately, then reassemble the firearm into its unimportable configuration: Taking that ugly thumbhole-stocked MAK-90 and adding the pistol grip, folding stock, etc after importation. This practice was perfectly legal until 18 USC 922r was enacted to close the obvious flaw in the legislation a year later in Nov 1990.
It applies to all semi-automatic rifles and and, according to a strict interpretation of the law's text, all shotguns regardless of mode of operation.
There are a lot of legal arguments floating around regarding the law's applicability:
Only ATF Import Branch has the authority to approve or deny a Form 6 import permit. If a firearm as been deemed unsuitable for sporting purposes and hence unimportable, then logically it has been submitted for approval for importation and been denied:
Does 922r apply to an assembled or reassembled firearm that has never been denied import approval? If the firearm in the same configuration has never been submitted for import approval, then its importability remains undefined for that configuration.
Can 922r apply where the resulting firearm is in a configuration not offered by the manufacturer, where there is no configuration to be denied importation? The Beretta Cx4 Storm is a good example where it is not a neutered version of a military weapon, but a standalone rifle made for the American civilian market. There is no manufactured version with a flash hider, non-thumbhole stock, folding stock, etc, so there cannot be an unimportable version."
Hope this helps.
BTW this SUCKS!!!