It's a crush washer... - Battle Arms Development Crush Washer

No worry's, no issues, works as it should and has kept the muzzle device snugly on the end of my 300 Black-Out barrel. For the fashion conscious you'll be very pleased to know that this crush washer (intended for use on a barrel with 5/8-24 threads) comes in your basic black which will likely match up perfectly with most muzzle devices now on the market. ;)