The Sylvan Arms Gen 4 AR-15 Folding Stock Adapter is compatible with 5.56 and 9mm pattern rifles and allows you to fold your stock for a more compact package for storage. Installation is even easier with the Gen 4, it simply screws into the receiver in place of your buffer tube. From there screw your buffer tube into the folding adapter. A great improvement for the Gen 4 folding adapter is that you now have the ability to install your take-down pin detent and detent spring via a screw port in the adapter itself. The Sylvan Arms Gen 4 AR-15 Folding Stock Adapter offers some other great improvements such as added QD sling attachment points, and a stronger and tighter lock up.