Check out ATF's wording in this December 15th letter. Specifically the part on page 2 that says: "Based on our evaluation, FTISB finds that the submitted forearm brace, when attached to a pistol is a “firearm” subject to the GCA provisions; however, it is not a “firearm” as defined by the NFA provided that the Blade AR Pistol Stabilizer is used as originally designed and NOT as a shoulder stock."
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