I have an approved ATF form 1 for a SBR (6.8SPC) with a 8" barrel, I've had it for a couple of years. I would like to get a 5.56 /.223 complete upper to put onto the reg. lower. Is this possible ? Most indoor ranges allow the 5.56 / .223 and the 6.8SPC can only be fired outdoors. What steps do I need to follow before I am allowed to do this. Also, the upper that I am looking at is described as 7.5". Would I need to keep it at 8" as the form 1 describes it as. Thank you for any information that you can provide to me.