When did suppressors become illegal in the USA?

Never, as far as federal law is concerned, though they are illegal in some states or areas.

The National Firearms Act (NFA) included silencers (the law uses that word, not the currently fashionable "suppressors") in its definition of a "firearm" that had to be registered and its transfer taxed. That law was passed in 1938. The NFA was included in the Gun Control Act of 1968 as Title II, but it is still commonly called the NFA.

HTH

Jim

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