ATF has made their guidelines well-known. There are four issues that could make a muzzle attachment "bad," depending on what rifle you put it on. Those are:
- It can't reduce muzzle blast (noise), or it's a sound suppressor.
- It can't reduce muzzle flash, or it's a flash suppressor.
- It can't be 22mm in diameter, or it's a grenade launcher.
- If it's threaded on, and not permanently attached with a BATF-approved method, it's a flash suppressor.
Kurt's brake takes all of these issues into account. It doesn't reduce blast or flash, it's 24mm in diameter, and he permanently attaches it using a BATF-approved (welded blind-pin) method.
Note too that a "letter" doesn't mean a whole lot. Tapco had approval for their brake, then a new person at ATF decided to redo the testing using totally different (and bogus) criteria. Suddenly, the brake was no longer approved, or something. DSA is taking legal action, and will probably get that decision overturned.
-Troy