That's pretty much answered in the "Significant rule changes at Williamsport" thread but I'll go ahead and answer anyway.
Brakes are allowed in 1000yd br in Williamsport and NBRSA matches on both light and heavy gun but only on light guns in IBS. They however cannot be of the clamshell type that directs the muzzle blast back. The same rules apply at 600yd matches.
