If you are talking about a SIG AMT there were no muzzle brakes intended specifically for them. There were a number of different types of flash hiders used. The flash hiders were installed usually by the importer so its a crap shoot. The most common were the FN49 type but I've seen a couple guns with HK types too. The threads are 15mmX1 I believe...or whatever the standard thread for the HK91. My rifle had the FN49 type. Many rifles had no flash hider. If you are looking for an actual muzzle brake you will probably have to get something made. Estes Adams made some threaded flashhider/muzzle brakes that duplicate the original 510-4 type. You might try contacting him if you want one of them.
hope that helps some.
Frank