That's not so much a lack of definition as it is Evike being misinformed. Several US retailers quote 500 fps as the upper limit, but it is not so.
The legal specs for it to be considered a firearm are 500 FPS AND 5.7 joules. Why both? A 300 fps paintball marker is in the 12-14 joule range. For airsoft, 5.7 joules is up in the 800 fps range and not usually something we need to take into consideration.
It should also be noted Evike sells non-airoft airguns (i.e., pellet guns). A standard .177 cal pellet weighs 8.4 grains (~0.55 g): quick math shows us at 499 fps it's already over 6 joules so anything above 500 FPS is automatically over the limit by a hefty margin. Maybe that's where the confusion lies (Evike tried shipping >500 fps airguns over).
But again, this is a lack of understanding of the regulations, not a lack of clarity.
Last edited by Drake; December 7th, 2015 at 00:53..