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.
And you could just use the front half of the threading as thread and the other half is turned 0.005 larger than the Thread diameter, and on the barrel the diameter is 0.004 over thread diameter.
That way you have a thread and 2 surfaces to align the tube to the barrel axis