Just to sum up in case anyone does a search and reads this thread down the road, the Sci Fi gun idea comes up at least a few time a year.
The folks in customs consider them to be replica firearms and will seize them. They launch projectiles and resemble real firearms. The very reason people want them puts them on the no fly list. the rules are left grey instead of black and white to deal with items like this when they appear.
There is no point in using circular logic to present a courtroom argument here that since they're from imagination land they should be allowed entry. The folks posting on this site don't control what gets into the country; we just use our past experience & the collective knowledge of the community to give new players advice. Point one is that those guns can't get in via loophole. Point two is a lot of them are low quality novelty items. Point three is that unless you have a full Aliens or Stargate rig you'll look and feel like a bit of a twat when you turn up at a game toting one.
The mechanics of how the hardware is imported is not ever discussed in the clear here. It is discussed clearly in the age verified sub forums. It's far to easy for someone to go off half cocked and try and order something from overseas that will be seized. Most of the big players know the score and will simply not accept your order. Some smaller shady folks though will take an order for something that will be seized. They don't care and they know that once the payment is made they're in the clear. The staff here don't need more psychopathic helicopter parents on here having a meltdown because little Johnny read something on here about ordering a gun and then had it seized and in the process lost his $200.
Airsoft, where nothing is hurt but feelings.