The decrease in recorded velocity as load limits are approached seems to be a common phenominum. This says you are in never never land and anything, any variant can cause a real pressure excursion. Your gun is trying to warn you!
I always document my chronograph and range data on Excel data sheets for each cartridge. This makes it simple to use the graphing function to display ‘MV vs powder charge’ 2-axis graphs. The slope tends to be fairly uniform but eventually tends to flatten as it gets near the charge where sticky extraction soon follows. I’ve learned, with time and hard knocks, to stop increasing charges when the slope shows a definite flattening.
I surmise that the point where the slope definitely changes represents as high a pressure as I’m willing to accept, without measuring chamber pressure more directly, at least with a strain gauge. Because I’m reluctant to do that, I accept the flattening slope as an empirical indicator that pushing charges - and therefore, pressures higher - doesn’t increase MV meaningfully and exposes the rifle and shooter to unwarranted risk.
I’ve graphed enough cartridges and rifles to rely on this method, despite lack of more direct evidence of chamber pressure, as good enough from a practical perspective. It’s kept my rifles and me in one piece, respectively.
