More powder,same velocity

[Downwindtracker2]

I have been doing load development for a couple odd calibers,264WM and 338.06.The rifles are rebarreled FNs ,one with a Douglas and the other a Shilen,both long throated,longer than then the 30.06 length boxs anyway.Their isn't a lot of data for them,so I have been shooting the groups over a Chrony.One time with each rifle,increasing the charge by a grain,<2%,The next loading shot the same velocity.What's going on here?