§ 1.4 gave the frame-or-receiver definition in summary; this is the working version, because every duty in this chapter attaches to the part identified here. A frame is the part of a handgun, or variants of it, that provides housing or a structure for the component — the sear or its equivalent — designed to hold back the hammer, striker, bolt, or similar primary energized component before the firing sequence begins, even if pins or other attachments are needed to connect that component to the housing. A receiver is the part of a rifle, shotgun, or projectile weapon other than a handgun, or variants of it, that provides housing or a structure for the primary component designed to block or seal the breech before the firing sequence begins — the bolt, breechblock, or equivalent — again even if pins or attachments are needed.

Variant and variants thereof carry more weight than they appear to. They mean a weapon using a similar frame or receiver design irrespective of new or different model designations or configurations, characteristics, features, components, accessories, or attachments. The regulation gives its own illustrations: an AK-type firearm with a short stock is a pistol variant of an AK-type rifle, an AR-type firearm with a short stock is a pistol variant of an AR-type rifle, and a revolving cylinder shotgun is a shotgun variant of a revolver. The point is that a manufacturer cannot move the regulated part by renaming or reconfiguring the weapon around it.

The hardest question is when an unfinished article becomes one. The terms include a partially complete, disassembled, or nonfunctional frame or receiver — including a parts kit — that is designed to, or may readily be, completed, assembled, restored, or otherwise converted to function as one. They do 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: an unformed block of metal, liquid polymer, or other raw material is not a receiver. Between those poles the Director may, when issuing a classification, consider any associated templates, jigs, molds, equipment, tools, instructions, guides, or marketing materials sold, distributed or possessed with the item, or otherwise made available by the seller to the purchaser. What comes in the box, and how it is advertised, can decide what the thing in the box is.