I hear of other people who've successfully brought these items over. It's a gamble. Eventually, you probably will have a package seized if you're a regular importer. There are many factors that can contribute to these items making it through, even though they were inspected.
Many customs inspectors may not know the exact letter of the law on airsoft, and figure it's just an airsoft mag, and let it though. And as you say, mopic, seizures are at officer discretion. If someone is feeling lazy and doesn't want to fill out a bunch of forms that day, he / she may allow the package through. An inspector who's in a foul mood may interpret the law very harshly and even seize totally legal items he / she believes are questionable. That's happened before too to other ASC members who've had legal parts seized, although it seems those occurances are pretty rare. It seems quite random. And of course, that doesn't include all the packages containing all these prohibited parts that made it through customs unopened.
A lot of stuff does get through our borders that (rightfully or not) should not be allowed into the country by letter of the law. However, many still make it through. But you are taking a change that these items will be seized and you'll lose your money. It's a gamble, but it's possible to get them.