http://www.atf.gov/firearms/nfa/nfa_handbook/chapter11.pdf
11.1.2 Firearms and firearm component parts subject to the AECA. As stated in Section 11.1,
firearms and component parts of firearms are included in Categories I and II of the U.S. Munitions List.
11.1.2.1 Category I. Category I covers nonautomatic and semi-automatic firearms to caliber .50
inclusive (12.7 mm), as well as fully automatic firearms to .50 caliber inclusive. This category also
covers silencers, mufflers, sound and flash suppressors, and rifle scopes manufactured to military
specifications. Excluded from Category I are non-combat shotguns with barrel lengths of 18 inches
or longer. In other words, sporting shotguns not subject to the NFA are not defense articles subject
to State Department export controls. Category I also covers components, parts, accessories and
attachments for firearms.
However, it excludes rifle scopes and sighting devices not manufactured
to military specifications, as well as accessories and attachments (for example, belts, slings, after
market rubber grips, cleaning kits) for firearms that do not enhance the usefulness, effectiveness, or
capabilities of the firearm, components, and parts. The Department of Commerce regulates the
export of firearms and parts not covered by Category I. Therefore, see the Commerce’s regulations
in 15 CFR Parts 730-799 for export controls over those items.