I think that 10fps loss/gain per inch thing is kind of a rule of thumb.
Further down this thread Paul indicates that he cut his down some 9.5" from the full length of just over 23". His velocity loss was @ 5fps per inch.
I too have found that I get @ 5 or 6fps loss per inch. BUT, I also adjust the gun to not be so wastefull with air on each shot. If the air isn't pushing lead, it is wasted. So, turning the power down a little keeps the shot count closer to before the chop, and brings my total velocity drop down to around 12fps per inch.
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