With the gun producing 3 out of 4 very good groups, (if at 25 M or more), the gun itself is capable, I would look at the loads. Start with new matched brass, uniform the primer pockets, trim the flash holes, trim the brass all to the exact same length and start playing with the crimp. If using a full WC bullet, you might want to use (recommend) a taper crimp, and I would set the crimp at .003". If you are using a RN or SWC bullet I would recommend a roll crimp and make sure it crimps just exactly at the base of the crimp groove of the bullet. I would start with a medium crimp and tighten it up about .002" each test run and see what works best.

Then look at the bullets you are using. Not all bullets are created equal. If using lead, (recommend), be sure and get from a best quality source. In paper punching loads, swagged seems to group best for me. As you increase velocity, cast bullets will probably be needed at some point (around 850 to 900 fps you may start getting a little leading with swagged). Make up 20-25 rounds of test loads for each batch, and when shooting shoot off a sandbag rest, and make sure the barrel doesn't touch the rest, only the frame.

Persevere, and you will find that magical load .

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H Richard
SWCA1967 SWHF244