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 know that muzzle pressure will vary based on cartridge, barrel length, projectile weight, powder and more. Wondering if someone has some ballpark numbers for: