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.
My preference in threading barrels is to have at least 0.050" wide registration shoulder where the suppressor meets the barrel. I have several hunting rifles that I would like to suppress, however the barrel is too small for a 5/8-24 thread. What are the thoughts on 1/2-28 for centerfire rifles? Is there a mag caliber that you would not consider for a 1/2-28 thread?