This mini, machined thread protector threaded in 1/2x28, is nearly 1.75" inches in length and bored out to accommodate the 9mm rounds. It has cuts in it similar to an A1 muzzle brake but they are not cut through to the bore. Complete with AR crush washer to prevent it from unscrewing under fire. Also fits standard AR 15 threads. (gun and other parts sold separately)
Approximation to protect seller's privacy