In order to have a variable Cyclic Rate, this gun can be taken apart easily to put a different strength spring in. A weaker spring would increase the Cyclic rate, but also theoreticly diminish end velocity. A stronger spring would slow the Cyclic rate, but theoreticly give a higher end velocity.
Also, this idea can be scaled up or down for any caliber. So it can be .177 BB, Airsoft BB, or even paintball. And if a large enough air source can be obtained, who can say, "Fully automatic golf balls!"

Also, the shaft from the piston may come back very fast, so if anybody builds this, you may want to shroud the shaft.
I plan to make mine out of copper, and chamber it for Airsoft. Now peter can't whore his CO2 powered fully automatic Desert Eagle.