Sometimes brass can be the culprit, weight sorting can help as often the heavier cases can have a lower internal capacity creating higher pressure. Brass from different lots can have a similar effect as well, neck tension and thickness can have an affect as well.
Could just be your chronograph playing up when clouds go by or if the light is not quite at the right angle, mine is hopeless in the late afternoon.
These are things that can make you start pulling your hair out. Settle on a load and go and see what it does at longer range because that will be the best indicator of what it will do in the field. If you can get out and set up some gongs, say one at 300 and one at 500 and validate your load you'll be away laughing.
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