Not if you stockpile imaginary weapons.
There are laws against stockpiling real weapons?
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Well, the question mark at the end gives it away.
But I'll ask it this way: suppose a bunch of people were stockpiling weapons and making plans to thwart an invasion of spiders from Mars. Is that a crime?
It is when they make an explicit plan to lure people into an area and kill them, like these guys did. Thanks for playing!
What people?
It is a crime to conspire to commit a crime. The guns and ammo would be corroborating evidence of the seriousness of the conspiracy.