QD sling swivel
Metal quick detachment sling swivel. Its purpose is to fit with a sling in to the standard groove, which you can find on a weapon (for exaple on the Tokyo Marui M40A5) or on accessories (some crane stocks, RIS mounts...). There are 4 bearings inside that fit into the groove and thus the swivel with a sling holds. You can release the swivel by pressing the button.