Okay, it's a technical thing.
But it's a real thing, too.
All bullets of the same weight and general design are NOT created equal. For instance, .452 diameter, 230 grain FMJ bullets are not all the same. They don't all shoot the same (this brand likes my pistols better than that brand, for some reason) nor do they all generate the same pressure curves.
Core lead alloys differ, jacketing material alloys differ, jacket thinkness differs, and a host of manufacturing processes cause different bullets to react differently in firearm barrels and therefore produce different pressure levels and curves.
This is not to say they aren't fairly close. Surely, starting loads across the board should be okay. However, if getting toward the upper end of the charge continuum, abruptly changing bullets is not a good idea. With normal handgun pressures, one will not blow up a handgun. Normally.
Even cast lead bullets differ in internal strength and rigidity. That's why these bullets go faster and those bullets lead the barrel more and them bullets won't group within minute of barn door. That's why when you and I load the same load from the same book with the same guns, yours is 75 fps faster than mine and both of ours is slower than the book said.
No, dagnabbit, they are not the same.
Close? Okay, fairly close usually. But they ain't all the same.
As long as I'm ranting, cases make a difference, too. The volume makes a difference we all recognize, but even the stiffness of the case mouth makes a difference in crimp strength.
By the way, guns are different, too. More on that later. But my model 15 S&W shoots faster or slower (with the same ammo) than your model 15 S&W. Not to mention how different my model 15 S&W shoots from your Colt Police Positive. Or your Taurus.
Start low, work up slowly and pay attention. If you add more powder and the velocity doesn't go up, that is not a good sign. Bad sign.
