You don't need to buy anything. Your gunsmith or machinist will use the same thread cutting tool he uses to cut everything from 1/4-28 to 1.062-16 tpi. A thread gauge will confirm the threads per inch. Most likely 5/8-24. Measure the thread relief inside the brake and it should be around .625" - .630" Cut the tennon .625" Set the lathe to 24 tpi and start cutting. Don't even think of trying to get a metric thread to fit. Once fitted concentric to the last 2" of the bore it will then need bored out to .020" over you bullet diameter. If none of this makes sense, take it to someone that it does.