End plate sling adapter securable to a rifle for a sling, includes a fastener element, sling adapter clamp and locking insert. Sling adapter clamp has a collar defining a central opening extending therethrough, and a pair of fastener housings arranged at respective ends of the collar defining a gap therebetween. Each housing receives a fastener therein such that the fastener extends across the gap and is adjustable to reduce its width. Locking insert has an upper portion with a threaded cradle accommodated within central opening and meshing with a threaded portion of a rifle buttstock receiver extension, and a lower portion extending into the gap.