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 have a form one can I'm working on should I do my end cap the same as the baffle bore which is 33/64 or should I do it .01-.02 over which is what I've read in a few places. What would y'all recommend?