UBBT's are perfectly legal (and the manufacturers are awesome people). They cannot be used when attached to the gun. It would be the same if you could unscrew your muzzle break and use that to drop mags, or use a bullet stored in your pistol grip.
Attached, its part of the gun, not attached, tool, and never the twain shall meet.
Attached, its part of the gun, not attached, tool, and never the twain shall meet.
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