R26 is such a good match with 150s in the standard 270 getting velocity around 3130 with Alliant data. I think you might have a little more potential with the AI. If your getting say 50-100 fps more with similar powders like R22 I would expect the same or better with R26. I need to double check but I am getting 140s to 3200 in the 280AI in a 25 inch barrel with R26. This was in hot weather and haven't re-checked them in colder weather yet.

Charge weight was about half a grain higher than yours. This is not lab tested so you are on your own regarding maximums.
R26 shows no pressure signs until you are way over pressure. Not sure if this is just the nature of progressive powders or not?

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