Used with special subsonic ammo. Rather simple design and easily made with a lathe. Only four metal parts. I wasn't aware of it until yesterday when I read about it in a book. Then off to the interweb where I found more from this site:
https://sovietarmorer.wordpress.com/2014/11/01/bramit-sound-supressor/
https://sovietarmorer.wordpress.com/2014/11/01/bramit-sound-supressor/