Thats it.
More power makes it an airgun and therefore a firearm.
Under 1 Joule means you cant have it unless its orange.
You can have it as a rif if you are a registered Airsoft Skirmisher theatre or film director or historical reenactor.
So to sum up if it is more powerful than 1 joule you can have one.
I think thats right.
Search VCRA airsoft for more details.