That's a very broad question. Are you talking rifles? Handguns? Shotguns? Excluding customs and semi-customs? What if part of the gun or the material is imported? Anymore, it's very difficult to say anything is truly domestic because very little of anything is entirely from any one place. Remington is one noteworthy in the purview of this site, but I don't know that their steel is domestic.
KC, not sure on your question so are you asking manufacturing plants on U.S. soil irregardless of imported components? If so that makes it a whole lot easier to answer, otherwise I don't know how anyone or at least myself could figure out what manufactures import what components on any given day with any degree of accuracy.