How much is your time worth versus the cost and difficulty of replacing a barrel?
See those little fins on the skirts they IMO are causing some of the problem. have you tried CPHP yet? I would also try the Crosman pointed as they seem to be a bit smaller. the barrel looks tight to me.
what is most accurate in my guns is when the heads have a light banding just enough that the head does not allow the pellet to pitch in the barrel. The CPHP as you know are harder and resist smearing fins on the skirts pretty good. I think that is why they can be decently accurate despite some variability.
also how is the leade is the transition to rifling abrupt with a hard edge or is it a smooth transition with no edge...
If I had any guess, its a velocity issue. But not in the traditional sence. As an experiment can you regulate the power down for the QB you are working on to shoot in the 500 to 600 fps range with HPA and the known pellets it shot well? Be interesting to see what happens.
No diamond ball just sandpaper and dowel, right...
Just thinking out loud. Does the barell have a choke? If not, why not give it one. That may help.
Also, did the previous owner enlarge the transfer port?? That could cause the pellet skirt to overhang the port and cause deformation on firing. That SEEMS the logical culprit if the gun was shooting better on Co2.Like I said, just thinking out loud.
If getting frustrated take a step back take a break and remember figuring these things and tinkering out is supposed to be an enjoyable challenge.