As pointed out the FFL refers to the dealer that must transfer the firearm to you. You would not need a FFL transfer for a slide but you would for a receiver as this is what the BATF considers to be the actual firearm.
In terms of citizenship status please refer to the link below. I do not know how valid or current the information is or whether certain restrictions have been put in place post 9/11 that are not reflected here.
US gun laws for non-citizens
"The whole of the Bill (of Rights) is a declaration of the right of the people at large or considered as individuals.... It establishes some rights of the individual as unalienable and which consequently, no majority has a right to deprive them of." (Albert Gallatin of the New York Historical Society, October 7, 1789)
"A free people ought not only to be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government." - George Washington