It should not be an issue for anything airsoft, but it is. The excuses we get are: velocity and how it looks (Replica laws).
Most parts, from everything I learned, are not a problem but even then... it depends on how the person checking the package reads their policies. Springs have been seized.
Last I checked, those laws/rules have not changed. So take your own risks.
Heck, you take a risk for anything imported by yourself. I prefer to avoid the headache.