27 CFR 478.12(a)(c) states:

The terms ‘‘frame’’ and ‘‘receiver’’ shall include a partially complete, disassembled, or nonfunctional frame or receiver, including a frame or receiver parts kit, that is designed to or may readily be completed, assembled, restored, or otherwise converted to function as a frame or receiver, i.e., to house or provide a structure for the primary energized component of a handgun, breech blocking or sealing component of a projectile weapon other than a handgun, or internal sound reduction component of a firearm muffler or firearm silencer, as the case may be. The terms shall not include a forging, casting, printing, extrusion, unmachined body, or similar article that has not yet reached a stage of manufacture where it is clearly identifiable as an unfinished component part of a weapon (e.g., unformed block of metal, liquid polymer, or other raw material). When issuing a classification, the Director may consider any associated templates, jigs, molds, equipment, tools, instructions, guides, or marketing materials that are sold, distributed, or possessed with the item or kit, or otherwise made available by the seller or distributor of the item or kit to the purchaser or recipient of the item or kit. 

All situations are unique, and a determination can only be made based on an analysis of the facts specific to the individual scenario. Licensees may contact ATF’s Firearms and Ammunition Technology Division (FATD) at fire_tech@atf.gov if they require a determination whether the items in question are regulated under federal law and regulation.