I did a quick search and found nothing saying that muzzle brakes are illegal.
It was NOT an exhaustive search.
You might want to read the following web page:
NRA Life Member
"War is an ugly thing, but not the ugliest of things; the decayed and degraded state of moral and patriotic feeling which thinks nothing worth a war, is worse. A man who has nothing which he cares more about than he does about his personal safety, is a miserable creature, who has no chance at being free, unless made and kept so by the exertions of better men than himself."
John Stuart Mill