Quick Detach sling mount

Quick Detach sling mounts (QD), also called Flush Cup sling mounts, is a type of mounting point used for attaching slings. "QD sling swivel" usually refer to the part attached to the sling, while "flush cup" refer to the corresponding socket on the firearm.

The system consists of a push button swivel attached to the sling (approximate diameter 9.4 mm, a little under $3/8$ inches) which is placed into a mounting socket on the firearm (approximate diameter 9.6 mm, a little over $3/8$ inches). The swivel is kept in place by a four ball detents connected to the push button, which presses against the walls of the socket. The sling swivel can quickly be taken on and off by pushing the QD button, which retracts the ball detents. The flush cup sockets can be either rotating or non rotating.

Flush cups can be mounted to a long gun in various ways, for instance by drilling a hole in the stock and screwing in flush cup socket, or by using an adapter to other mounting standards such as Picatinny or M-LOK.