I've read everything that has been written in this thread, but I think everyone is talking about different things.
I am not a lawyer, and I'm only working off of the information I have learned thus far in my life.
It is not illegal to have a threaded muzzle on a rifle (in most states)
Nat isn't saying that threading a muzzle is illegal or that suppressors, silencers, cans, anything else you want to call them are bad
Nat is saying that the act of threading isn't the problem, it is potentially being an accessory to a crime, aiding and abetting, etc.
What I do know is that the government can and does take away people's licensures for reasons that don't make sense. Those people have to fight in court to get them back. That court fight (in order to regain the ability to work and make a living) is extremely expensive.
Likewise, the BATFE has had a historical event (thank God it isn't a common occurance) wherein they convinced a guy to do something legal (that is what the court found), but they made an armed assault on his home. That event started with them engaging his dog with automatic weapons fire. That fire was so close to his son that the boy returned fire, likely out of fear, and was shot to death by a federal agent with an automatic weapon. The guy's friend, now engaged in a lethal firefight, mortally wounded an agent. I believe that the guy was also wounded.
Later, the guy's wife was shot and killed by a sniper when she was unarmed and simply holding the door open. She died in front of her remaining children.
Now, the guy was innocent of the act that BATFE charged him with, but that "legal" act cost him the lives of one of his children, his wife, and his dog, it cost the life of an innocent federal agent that was fed bad intel, and it cost untold money, time, and the man's own livelyhood.
Am I saying that the guy was a saint? No, I don't know him, and I don't know his mind or intent. I am only pointing out that doing something legal while discussing something potentially illegal can have bad consequences.
I'm thinking that "legal" doesn't get it done. Careful, extremely rigid toward legal issues, and double checking with the regulatory agency before acting might just be prudent.
If you believe otherwise, good, go do it. There likely is nothing illegal about it. I would just suggest that everyone looks up once in a while. There are things above your line of sight that you aren't seeing.