Does your gun have to be below 330fps? What if it is rated exactly 330fps? Is that ok? I want to get an mp5 and its rated at 330fps.

330 FPS is the limit.

You can have a gun that shoots at 330 but that also means the gun will move around a bit due to weather conditions and manufacturers are not always truthful about where there guns are at and what they actually shoot at, you can also get a spring and have the inhouse tech swap the spring for $40.00 with about a week turn around.