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.
I made a youtube video on single point threadmilling. I'm a huge Bobcad user and fan, but Advent 2008 is free and does a very good job. I did an AR-15 lower in Titanium and thread milled the buffer tube exactly the same way I did it in aluminum in the video below. I just adjusted for the correct feeds and speeds.
