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.
M1, check out ducktape videos (25dollar can) viewtopic.php?f=10&t=81439 looks like his AR is cycling, I might be wrong though. I think he also was inspired by subsonics builds.