Did you fire the followup rounds immediately, or had a day or two lapsed? If the rounds that your fired were done so consecutively, and if it were a false high, whatever influenced the false reading would still be present.
If done later, and the chronograph had been broke down and reset up and weather had changed, then I would most likely eliminate chronograph error as a cause for such a huge jump.
How was ambient temperature between strings?
Were the one-grain-higher charges from another lot of RL15? Even so, that's a big jump.
Let me answer your question and see what you think:
I fired the follow ups with the 58 grn charge before I left the range. In fact I was already packing up then I had the thought to do that to rule out something with the chronograph.
The ambient temp was probably in the 70s. It was sunny but I was shooting under a covered bench and the chrono was set up in the shade.
If flattened primers aren't a good sign of pressure and the bolt lift was easy, what else do I look for?
