Picked up a used P17 and fiddled with it to see if I could squeeze some more power out of it, starting with the following modifications:

1) cleaned up the valve stem, installed a harder o-ring and drilled out the transfer port to 3.8mm from 2.5mm

2) added a Delrin sleeve to the chamber to reduce volume and increase compression

3) machined an old Crosman barrel to fit at 9.75"

https://i.imgur.com/jy6kICT.jpg

https://i.imgur.com/7bwz7jT.jpg

Here are the results:

https://i.imgur.com/hoXSrJs.jpg

Incremental increases in power except for the increased compression that actually reduced performance, contrary to some results reported elsewhere.

My original intention was to convert it to 0.22" and rather embarrassingly that's what I thought I was doing with the long barrel, in my mind it was a 0.22" I had in the scrap box and I didn't bother checking

I did however have a spare Crosman 2240 barrel and machined it to fit, here it is between the original and the long 0.177" barrels:

https://i.imgur.com/jNu8pg8.jpg

Test data for the 0.22" compared to the original 0.177":

https://i.imgur.com/xSDKOCP.jpg

25% more energy and pretty consistent.

I also did a test comparing closing the pump lever slowly and quickly, the latter is slightly more powerful and consistent:

https://i.imgur.com/jtuq1Wu.jpg