Ok ! Let say, what you can not import would be better and easy to let you understand.
There are a few part that you can't import like lower receiver, silencer or suppressor, the whole gun, grenade or M203 that look like real and magazines that had dummy rounds like P90 mag. That would be more than that but at least, it gives an idea for what can and what can't. Please do a little bit research on the FAQ, there should be more specific to cover for what I don't list yet.