I recently purchased a Mac 10 submachinegun and would like to compile a list of upper receivers. This is my first NFA item and I am going with the trust route To avoid Complications with my 18-year-old brother. I would like to buy as many Different uppers as possible. I was also wondering if a semi automatic upper will work on a fully automatic lower receiver. PS I am having them put me on the list for Lage upper receiver I just haven't decided if I should go with the .45 ACP or 9mm first.