I know that the KA M4's that are based off the WA GBBR design just got approved for import by the BATF.
They're kind of weird like that....
@ shadow: The WE KAC PDW (Open Bolt)
gets some pretty good ratings. You should be fine with that.
Otherwise source out an Open Bolt M4 and put the right furniture on it to turn it into the MGS4 M4A1.