Loading for short-barrel AR type rifles is a pain because loading data is all over the place for barrels of substantially varying lengths and with varying twist rates. For almost any loads approaching max in any gun, I find that the chronograph tells the tale when doing a load workup. There usually comes a point where increased powder results in ever diminishing increases in velocity, or even decreases, and the extreme spreads and standard deviations start rising. I back off at that point, then test the more consistent loads for accuracy even if the velocity is not the highest. Of course, you also need to look for standard pressure signs like flattening primers, ejector marks, extraction, etc.
