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."
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Here is my take.
Id say that the LAST thing we need to to is swamp the ATF with letters regarding the legality of our imported shotguns.I like my new M4 with colapsable stock and +2 extension. The last thing I want to run across is some local jerkoff LEO that knows 922 inside and out because the ATF found need to make an issue of it.
Out of sight, out of mind. If everyday, the ATF is getting 3 letters regarding 922, they are damn sure going to brush up on what it means, and pay it more attention.
I have yet to meet anyone, or even hear of anyone on the net that has been nabbed for running a Benelli with an extension.
So, my thought is to do what you want, at your own risk. We know what the law is for the most part, lets not push it.
Id say that if you are worried about modifying the M4, then dont. SImple as that.
That being said,
I myself have a new M4, with all the evil parts, and I plan on a mag extension. A few guys from ARF.com got in contact with the ATF. They reported that the ATF said that unless you use the gun in a crime, you dont have anything to worry about. One of them said "Just dont use it to hold up a liquor store, and your good" Those exact words.
I see wayyyyy to many modified Benellis out there at local 3-gun matches for it to be an issue. And I highly doubt that any of them have it registered as a SBS.