Last night I did a little experiment that I think showed I should not open up the porting to 0.17 as it would make the string to peaky for my goal. My transfer tube blew out (see pic) so I installed a piece of ice maker tubing with 0.17 ID and the string had fewer good shots than with the 0.157 tube (39 shots with 0.17, 47 shots with 0.157). Efficiency did go up a bit. Both strings have 3 o-ring BHB.I don't understand why just replacing the transfer tube caused this difference when the valve exhaust and barrel porting were still at 0.157.Gonna try sleeving the ice maker tubing with brass rod I can drill out.
restricting the airflow path decreases the usable pressure range
it definitely won't increase the pressure range over which that lower velocity can be achieved.
Conversely, opening up the ports inevitably leads to a "peakier" power curve with a narrower pressure range, just like using a hot cam in an engine....