Yes, it can be legal to make a silencer. For everything Form-1, from silencer designs that are easily made, to filing forms with the BATF, to 3D modeling. Remember, you must have an approved BATF Form-1 to make a silencer. All NFA laws apply.
Does anyone know if there is a manufacturer that makes a QD adapter in 1 5/16-24? I have a TMSA multi-cal I would like to make more versatile. Any info is greatly appreciated!