The spacers don't really have anything to do with velocity. They fine tune the length of the powertube on a micro scale, not a macro. It's more for setting up the relationship of the bolt piston (the rod inside the bolt) to the length of the powertube for more reliable operation. I think that the spacers are graded in thousandths.
In my experience the .220 is the most often used spacer (I don't recall off the top of my head what color that would be if you are using a benchmark set... red, I THINK), and you can adjust up or down from there to get the right fit. Since you aren't cycling with a barrel on I would guess that yours is adjusted with too big a spacer, and you need to go down.
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