Since this is in the NFA section, I assume you mean you want to re-activate a full-auto firearm? If that is your intent, then it is not legal, not by a longshot. (uless it is a registered DEWAT, and if so, you would already know that) If you mean you want to reactivate an otherwise legal semi-auto firearm, then you can do that all day long. Please clarify your intent so that we can be of more assistance.