My experiences with 9mm indicate that not all FMJ bullets of the same weight are created equal. Two bullets, each from a different manufacturer, may be of the same weight, but a difference in bullet profile to a specific overall length could result in a too-deep seating of the bullet, thus playing hob with pressure. Better, I think, to follow the recipe and use the same bullet to the specified overall cartridge length and, as always, work up. It helps keep variables under control.