if they shoot over 500 fps and the maximum prescribed energy (can't remember the number offhand), then that classifies the weapon as a firearm. Since they're fully automatic, if they get classified as a firearm, more than likely it would results in a charge of possession of a restricted firearm. Don't think there's anything to worry about though. I'm pretty sure an airsoft gun would have to shoot a .2g BB WELL in excess of 600 fps to consider it a firearm.
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