Had a little time with pistol today. It gets dark early
No re-barreling will be needed. With hammer spring almost all the way in it produce with 14.3gn Crosman HP 680-690 fps.
At 15 yrd they still give almost an inch group.
Things changed then I tried again 18 gn JSB (5.52). One hole groups (literally) at this distance for 24+ shots. After that I was loading one by one another 5-6 pellets before the POI moved down about 1/4 in. I didn't crony it with JSB 18gn yet, but POI is about 1/2 lower then CPHP 14.3gn at 15 yrd.
I will still take it a part and tune a bit, but now it looks much better even with expensive appetite. Longer shroud is definitely in order as well.
Edit: JSB 18gn shot at 630 fps with the same settings.