Seems super vague
"accessory that is designed or functions to accelerate the rate of fire of a semiautomatic rifle but not convert the semiautomatic rifle into a machinegun."
So trigger with different pull weights?
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Ambiguous rate of fire -- what's the rate that qualifies as machine gun?
Ambiguous use of "accessory".
Bump-fire stocks are non-standard, after market replacement parts. "Accessory" implies something added to the original rifle, such as a light, optic, etc.
a thing that can be added to something else in order to make it more useful, versatile, or attractive.
"a range of bathroom accessories"
synonyms: attachment, extra, addition, add-on, adjunct, appendage, appurtenance, fitment, supplement
"camera accessories such as tripods"
A binary trigger would be a replacement part, not an accessory.