Just as title says, 1/2-28 to 5/8-24 thread adapter with thread protector, going from 5.56 up to 7.62 threads, so you can run 7.62 cans, flash hiders, on a 5.56, so you can run anything on a 5.56 AR from a 7.62 AR, M1A, it's from brownells sn# 100-009-345 made by Trosusa, got it last month, just sold the AR, this didn't go with it, still have orginal packing, send private message, $45 shipped