any part, combination of parts, component, device, attachment, or 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.

Sen. Dianne Feinstein (D-CA) and Rep. Carlos Curbelo (R-FL26), S. 1916 (115th): Automatic Gunfire Prevention Act

So any change in parts that the government could argue even very slightly increases the rate of fire, even a 0.1% increase, would cause the semi-automatic gun to be banned. Our guess is that if the Democrats again try similar legislation, nothing will get passed.

A copy of the Supreme Court decision is available here. Justice Thomas’s first paragraph clearly states the argument.

Congress has long restricted access to “ ‘machinegun[s],’ ”
a category of firearms defined by the ability to “shoot, auto-
matically more than one shot . . . by a single function of the
trigger.” 26 U. S. C. §5845(b); see also 18 U. S. C. §922(o).
Semiautomatic firearms, which require shooters to reen-
gage the trigger for every shot, are not machineguns. This
case asks whether a bump stock—an accessory for a semi-
automatic rifle that allows the shooter to rapidly reengage
the trigger (and therefore achieve a high rate of fire)—con-
verts the rifle into a “machinegun.” We hold that it does not
and therefore affirm.

MERRICK B. GARLAND, ATTORNEY GENERAL, ET AL.,
PETITIONERS v. MICHAEL CARGILL

However, I have to say that I was pretty shocked that the liberals on the court, in their first paragraph, conceded that AR-15-type guns are “commonly available, semiautomatic rifles.” This seems to be a huge concession that will undercut the efforts by Democrats on lower courts to redefine Heller to mean commonly used in self-defense. I suspect this concession will appear in all the briefs challenging state assault weapon bans.