Are you referring to flash or sound suppressors?
For a long time ATF has considered to be contraband items like modified washers designed to be inserted within a suitable silencer body when both the washers and body are present, but now they're even talking about extra suppressor mounts with different thread pitches designed to fit various barrels to be a regulated item as well.
In the community of NFA collectors, there's some people who can show earlier and current ATF opinions that this position is invalid, some day that it doesn't fit their own legal definition of a regulated sound reduction device others insist that it's been retracted, and others say that it's still in effect even though the ATF hasn't swooped any manufacturers up yet. Also, some claim it doesn't matter what the ATF's opinion is because they can't just invent an imaginary law. Others who are in jail for running afoul of the ATF say "Oh, ho ho yes the hell they can!".
What nobody wants to do is go to ATF and ask for clarification: With those people, you get a different answer every time someone new picks up the phone. It always makes things worse because they immediately default to 'No, that is illegal and we will imprison you forever' mode, and that's not just some 'when in doubt, the answer is NO' doctrine they've adopted. Even when they know they're wrong they still say 'No'.
It's like asking the most nervous panicky liberal-assed mother in the neighborhood if you and your friend can build a treehouse in her backyard.