"OAL on the brake is 3.438", and max internal threaded depth is 0.850". Even if the 13.5" barrel has a threaded end that's 0.850" long, the brake will still add 2.588" to the barrel length, when permanently attached. That's a 16.088" overall length.
That's only a RCH over 16......ATF only uses wooden yardsticks for measuring...sooooo