More failure and some success.
It has been dead calm the last couple morning so I have taken advantage and done some more 300 yard testing.
I tested 3 more batches of the Bergers and things only got worse from the last 1.8" group that I shot. I just can't seem to get the consistency that I want from these within the pressure limits of the gun.
I decided to take a quick detour and try some 87 grain Hornady V-max bullets I had picked up. I was hoping I would have an easier time finding an accuracy node with the lighter bullet before I got to the pressure limits ... in theory. The trouble I had was finding consistent book load data to work from. The Hodgdon website gives load data for H-4831sc down to 90 grain bullets, but not below. That should be close enough. Hornady does not list this combo but the Berger book does show the powder for their 87 grain bullets. Of those two sources, the minimum starting charge was 4 grains different. When I reasoned that this bullet is only slightly lighter than the Berger I was using, and this gun seems to run hot, I decided to go with the lower Berger book numbers.
I loaded up three test batches in 0.5 grain increments from the book minimum using a seating depth of 0.025 off the lands. This should be a good general starting point. These loads were of similar charges to what I had been putting under the slightly heavier 95 grain bullets. I also made note of the expected velocities. Book minimum, corrected for my barrel length, should have been around 2821.
With yesterday morning's perfect weather I set a test target up at 300 yards and got on the new bench I built.
First group was the book minimum. It shot very well, but with an average velocity of 3014, almost a full 200 fps over expected. I also shot the other test batches, but neither beat the first group.
The next step will be load up a few more batches to see if I can reproduce these results. Given the gun seems to generate such high pressures, yet shoots better on the lower end, I will also try different seating depths. If I seat the bullet longer, yet still short of the lands, I might drop the pressures just a fraction more.
Overall, this is still a learning experience for me, but I think I'm getting somewhere with this one.
P.S. This is just a 4-shot group. It was intended to be one cold bore fowling shot and then a 3-shot group, but the fowling shot (left most one) was within the group so I included it.