Awesome! It's great to see something I did 4.5 years ago has found itself reincarnated (sorta) with you. Your version seems to be a much better design than my ghetto original.
I had a hard time with the "isolation" valve because at higher pressure, it was harder to seal and pull backward. The trigger needed alot of forward pressure to work. Too much actually, and instead of using enough rubber bands, I actually had to give the trigger a little forward nudge with my finger in order for it to work.
And I guess I had the "true" semi-auto down as far as the valves worked, but I still had to manually rotate the 2K turret. I see you've got an actual semi going on.
So yeah, in a nutshell, great job! Hope everything ends up working for ya.
The Sneakshot is a really great old gun. If you remove the old barrel, and glue a CPVC one on, you can get about 80ft. Its shell is big enough to integrate other stuff on it. With the two Sneakshots I got ahold of, I integrated a 2K onto the front of each. You use a keyring on the firing pin as a trigger.