For my electric repeater XS60C modification, I was originally going to use a motorized "chain gun" system like a lot of electric airsoft guns as a reloading mechanism ("hammer" is an electric solenoid, see http://www.gatewaytoairguns.org/GTA/index.php?topic=103085.0). Other than using a solenoid as a hammer, using my own bolt, and sawing off the material behind the valve where the hammer and bolt used to be, is an otherwise stock XS60C CO2 version. I decided to put the new bolt I crudely fabricated alone into the breech (after loading a pellet) and pulled the trigger. The result was both the pellet going down the barrel and the bolt being ejected backwards, clearing the breech. I did the same with the stock XS60C bolt and the same thing happened. This gave me an idea for a pure blowback reload system like you see in "real" pistols and submachine guns. The first picture is my 3D printed prototype. It's a little finicky due to the geometry of the parts and because I miscalculated the guide rod length and the plastic part of the bolt's geometry. The second picture is a single screenshot showing the spring loaded bolt clearing the breech area after a single shot was fired in the testing rig. I wish I could export the whole video, but I had to use the SloPro app since my iPhone's default SloMo camera wasn't quick enough to capture the cycle. The bolt does return back to into the breech about 70% of the time, again due to my mistakes in its geometry. Blowback only happens if a pellet is in the chamber which generates enough pressure to cycle the bolt. If nothing is loaded, the bolt will not fly back. So a blowback system does work for reloading alone. I don't think the air has enough energy to pull back a conventional coil spring loaded hammer like what is on the XS60C.
The SMG .22 from Airordanace maybe what you need to look at.Hooked to 1250 PSI HPA it will do 800+ FPS.