The US District Court in New Hampshire has ruled in the case of Sig Sauer v. BATFE that:
“The ATF’s classification of Sig Sauer’s [muzzle brake] device as a firearm silencer was not ‘arbitrary, capricious, . . . or otherwise not in accordance with law.’ 5 U.S.C. § 706(2)(A). Accordingly, I grant ATF’s motion for summary judgment (doc. no. 18), and I deny Sig Sauer’s motion for summary judgment.”
The brake doesn’t reduce the noise of the gun at all, but since SIG planned to make and sell a specially designed shroud that would transform the muzzle brake into a properly registered silencer, ATF banned the brake and Sig sued.