An adapter is provided with a rail having one end secured to a gunstock; a groove structure including a bottom groove, a forward socket, a transverse channel through the socket, a pin hole through the channel, and a bottom limit member; a lever pivotably secured to the groove structure; and a sliding member in both the channel and the socket and including a projection at one end, a shoulder adjacent to the projection, a receptacle at an other end, a spring in the receptacle, and a slot perpendicular to the receptacle and communicating therewith. The pin is through the pin hole and the slot to anchor the spring between the pin and the receptacle. In a locked state, the shoulder is in the socket to lock the rail in the groove, the other end of the sliding member is projected, and the projection is retracted into the channel.