Designed for use with the Maple Leaf Crazy Jet or their AEG barrel's, however they can also be used with other barrels like ZCI.  Inside of the bucking there's a circular rubber inner ring, just push the bucking on with a little bit of force.  An excellent bucking that gives an extremely tight fit onto the inner barrel.
Inside of the bucking there's a much larger contact pad.  To get the best out of the ideally use with an Omega nub.