After I made my statement about the Boberg, I did some checking and found this video of Arne Boberg firing the XR9-S without a recoil spring, he calls it "flip firing":
http://www.youtube.com/watch?v=vbmVjvlDPK0
Quote from Arne:
http://www.youtube.com/watch?v=vbmVjvlDPK0
Quote from Arne:
the XR series does not use a recoil spring... the spring that runs the length of the slide is simply a slide return spring. you could toss it in the trash and shoot the pistol the rest of your life, you would just need to slap the slide forward manually.
in the XR series, the recoil is being absorbed by the lift mechanism and the rotating barrel mechanism. these are FAR more efficient at absorbing energy than a simple heavily packed recoil spring and a bunch of extra weight on the slide could ever do. that is how we are the softest felt recoil .45 and 9mm subcompact out there!"