Modeled after the Q Sling Devil, you use this in conjunction with some Paracord to attach slings (or other items) to your firearm. Works with airsoft, real firearms, or even… chairs?

To tie the open ends, I used 18in of paracord for each, and tied a Double-Fisherman's knot to secure it to the sling. (Picture above, video below on how to tie it) Someone has tested the original design to 120LBS pulling on his sling without failure.

Here's the original video that shows it off, although the video is pretty insufferable.
 

And here is the video that I referenced to tie the knot to keep the paracord attached to the sling: