Here's how it breaks down. If the brake is removable (threaded barrel), then the OAL to the end of the threads must be at least 16" regardless of how much length the brake adds when it is screwed on the barrel. However, if the brake is permanently attached (red or blue loc-tite does not qualify as permanent), the you measure to the end of the brake to get your OAL. As long as its over 16", you are OK.
But, you can register the frame as a short barreled rifle, and legally use any barrel length you want. However, this takes about 3-4 months, a CLEO sign off, and $200. Oh yeah, you have to live in a Class III friendly state!