Your first 10 shots are still starting low and climbing. An educated guess suggests that you might have a creeping reg or perhaps a slight leak between the main tube and the plenum. The higher pressure at the start of the string will cause the lower fps. If the reg was working properly, you wouldn't get that rise when shooting a nearly full tube. The first two shots in each string are marginally lower so perhaps there is a little bit of creep going on.
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