My local gun shop. ShootSoCal Firearms has a legal muffler on a 300 Blackout short rifle.   He is a gun dealer and is allowed to do so.  With this in mind, I plan on putting out a card table in front of my hosienda with guns for sale. I sold a gun over 50 years ago and still mourn the loss.   Dave, the owner of SSC firearms, laughs at my cleverness. 
 An associate made a muffler for his 9, and it was very quiet. He put it in a briefcase, then left it somewhere, along with his I.D.   30 years ago, which cost him 15 grand to keep out of the pokie.