Most, if not all of the major ammunition manufacturers do not use the commonly available "cannister" powders. They buy bulk lots from the powder makers, and tailor the loads to what powder they have bought. They can specify a certain type, ball, flake, extruded, with a preferred burning rate, but will have to "work up" a load depending on the particular characteristics of the powder received. In a nutshell, they aren't using the powders we use. The cannister powders we buy are kept to very tight specifications, so if we buy a pound of Reloader 15 today, and another pound of Reloader 15 five years from now, it'll be very close to the same. The ammo makers don't buy 100 pounds at a time, they buy a ton or 2 at a time. They are oftentimes using powders we can't get.
On a side note, years ago when I was shooting IPSC, I was using a S&W 559 9mm. (Held more bullets, you know). I was loading 6.2 gr of Unique and a magnum primer with a Speer 125 gr JSP, and getting 1270fps over a chronograph. It made "major" on the IPSC power scale. I'm pretty sure it would beat up most guns, but the old S&W just ate that stuff up. It was the 9mm load that would clear a bowling pin off the table. I shot thousands of rounds through that gun, still have it today.
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