I need help with an up coming project. I would like to order a short barrel upper from CMMG 10.5" but I'm not sure how to start the process. Do I have to register with ATF before I try and purchase the upper? Can I order through a local dealer or does it have to be with a class 3 dealer? Are there any other legal issues with this project? I live in Pennsylvania and machine guns are legal here. It would be a semi auto short barreled rifle when complete. Thanks in advance for your help.

www.ar15.com/forums/topic.html?b=3&f=14&t=178305

Check out the SBR FAQ that edwin247 linked to above for the whole process.

But to give you the short answer: yes, you'll need to get your approved SBR paperwork back before installing a short barrelled upper onto your lower receiver. You can buy the upper directly without going through a class 3 dealer.