So I've worked up dozens of loads over the years in good number of rifle calibers. From 300blk to 7.7 Jap. I've seen this phenomena exactly twice and I am baffled at how to explain it.
While working up a new 7.5x55 (7.5 Swiss) load, I loaded 15 rounds each of 3 charge weights. For safety, I actually started at the starting load and fired a few rounds to work up to where I want to start accuracy testing. I did this to look for any pressure issues and to check that velocity was where it should be. The loading behaved as it should based on published loads/velocities, so I moved on to the 3 test loads for accuracy. I found a definitive accuracy node at about max published load -2%. Chrono data told me velocity is right where it should be. The load was 41 grains of IMR 4064 under a 180gn Hornady SP. While the other loads (38 gns and 39.5) shot high but center of windage, my most accurate load held a nice tight group about 5 inches left of center. And by nice tight group I mean all 15 rounds were under 3", but this shooting 3 separate groups. The largest individual group was a smidge over 2", the best being a bit under 1.5".
The main question is, have you ever found a load during your workups that inexplicably had a significantly different windage POI than the other loads? I can understand elevation changes in POI during workups, but not windage changes. This is the second time I've had this happen, the other time was finding a 300blk load that my son's rifle liked. In both cases that I've observed this, the most accurate load showed a significant difference in windage POI than all other loads. Every other workup I've done proceeded like normal. Minor shifts in POI, but nothing that raised eyebrows. Shifts of an inch or two at most. Not a pronounced 5" shift. What could cause this?
While working up a new 7.5x55 (7.5 Swiss) load, I loaded 15 rounds each of 3 charge weights. For safety, I actually started at the starting load and fired a few rounds to work up to where I want to start accuracy testing. I did this to look for any pressure issues and to check that velocity was where it should be. The loading behaved as it should based on published loads/velocities, so I moved on to the 3 test loads for accuracy. I found a definitive accuracy node at about max published load -2%. Chrono data told me velocity is right where it should be. The load was 41 grains of IMR 4064 under a 180gn Hornady SP. While the other loads (38 gns and 39.5) shot high but center of windage, my most accurate load held a nice tight group about 5 inches left of center. And by nice tight group I mean all 15 rounds were under 3", but this shooting 3 separate groups. The largest individual group was a smidge over 2", the best being a bit under 1.5".
The main question is, have you ever found a load during your workups that inexplicably had a significantly different windage POI than the other loads? I can understand elevation changes in POI during workups, but not windage changes. This is the second time I've had this happen, the other time was finding a 300blk load that my son's rifle liked. In both cases that I've observed this, the most accurate load showed a significant difference in windage POI than all other loads. Every other workup I've done proceeded like normal. Minor shifts in POI, but nothing that raised eyebrows. Shifts of an inch or two at most. Not a pronounced 5" shift. What could cause this?