I have a SIG556 Patrol Rifle that I want to SBR. Because the barreled upper is the actual firearm on the 556. After I send the Form 1 off and get my stamp back. How do I go about getting the barrel cut down? Since it obviously the work must be done by a gunsmith to do it right...Do I have to be present when the barrel is cut down or are there gunsmiths that can do it without my presence?