That’s not exactly true, my 40. caliber flintlock shoots best with 40 grains of 3f with a .015 pillow ticking patch, while my Lyman GPR caplock in 54 cal shoots better than I do with 60 grains and a .015 pillow ticking patch. There is such a thing as overloading the rifle. When I deer hunt I’ll bump my 40 up to 60 grains and my 54 up to 70 grains. I try to stay close to my most accurate load and still maintain a lethal load for deer.
My experience tells me that that if I overload my rifles, then I risk burning through the patch before the load leaves the barrel…. Therefore reducing accuracy.