7606. Entry of premises for examination of taxable objects. (a) Entry during day. The Secretary may enter, in the daytime, any building or place where any articles or objects subject to tax are made, produced, or kept, so far as it may be necessary for the purpose of examining said articles or objects. (b) Entry at night. When such premises are open at night, the Secretary may enter them while so open, in the performance of his official duties. (c) Penalties For penalty for refusal to permit entry or examination, see section 7342.
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Note that this definition is only in the rules for the NFA, and not the GCA. It is designed to interact with the AOW definition. For example even though this definition excludes such things as the .410 T/C Contender pistol from the pistol definition, it is also not an AOW as it has a rifled bore. And it is also a handgun under the GCA. The NFA statute does not define “pistol” or “revolver”. I think that excluding handguns designed to be fired when held in two hands is not necessarily justifiable. But it allowed them to declare that an HK SP-89 pistol with a K grip is an AOW. As is an M-11/9 or TEC-9 with a foregrip. The Auto Ordnance 1927-A3 pistol is apparently exempted, for historical authenticity.