My play with GRT didn't want to match up either. I ran the OBT calibration which changes burn rate or something to make predicted match actual and that made things fit a lot better, but also reckoned they are compressed loads at close to (or above) max pressure.
Have you tried lower charges? The Optimal Barrel time function reckoned there may be a node at 25.3gr of 8208 at a mv of about 2609fps.
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