Freedom of speech is not unlimited. Here, the government would argue that providing access to an illegal weapon mod falls within the “imminent lawless action” test, which allows the government to restrict speech if the person speaking intends to incite a legal violation that is imminent and likely to occur. I think it would be hard to argue against the governments position on that one considering full auto weapons are illegal, and use of this file to create a full auto weapon is likely.
To be clear, I am not advocating for the government to do this. I’m just thinking through the worst case scenario.