SEHunter,
There's really no fixed answer to this. As Frankenmauser explained, when the powder is the same powder, such as, say, IMR 4895 used in a commercial load, it will be usually just be bulk grade instead of the narrower burn rate canister grade. The reason is simple: manufacturers load with pressure guns that let them adjust loads to specific parameters, and while a small minority of handloaders have pressure test gear, too, most have to work only from recipes. The only way to keep recipes valid is to use a more tightly controlled powder burn rate, which is what canister grade has. This is made by the manufacturer who, whenever a bulk lot of 4895 comes out extra fast or extra slow, holds some of those lots back to blend with future lots of 4895 to correct their burn rates to canister grade tolerances.
But that isn't the only thing going on. For example, Federal gets special IMR4064 made with additives for flash suppression for making Mk.316 m.0 sniper ammo for the military. Some of Hornady's old Light Magnum loads used an odd elastic powder that was so heavily compressed it would gradually expand and overflow a case when you pulled a bullet from it. The handloader just doesn't have tools that will let him get enough of that sort of powder into a case.
The idea that ammo or powder manufacturers blend different powders is, AFAICT, a myth. The powder manufacturers blend different lots of the same types of powder, and some kinds of spherical powders have pretty wide particle size ranges, but I've never seen, for example, a stick powder with two different diameters or lengths of sticks in the same container. The reason is that stick powders settle quite a bit with vibration, where spherical powders don't, and the result is that you could expect to see stratification with transportation vibration of a blend of different stick sizes, and that would give you different burn rates for different parts of a container of powder. If it happened in loaded ammunition it would change the pressure curve. So, AFAIK, they only blend lots of the exact same type of powder, as I described. The blending also has to be proved to be adequately homogeneous, and pretty much, only the powder manufacturing factories have blending and testing setups for doing that. I found that out by talking to Hodgdon if they published powder properties like those in QuickLOAD, and they told me to get all those numbers they would have to pay a lab about $50K a pop. They just distribute what they order to a certain specification from the powder plant, leaving the the powder maker to work out how to get it there.
Another board member told me one time that he had pulled down Remington Core-Lokt ammunition (now a
77 year old bullet design) he'd purchased at different times over several decades and at various times found stick, ball, and even flake powders in the same bullet weight for the same chambering. Because the manufacturer has pressure test guns, he can adjust the load of any powder to a particular test gun velocity or peak pressure. He then only has to verify that the powder chosen produces both within acceptable limits. But you can bet there will be a tolerance of some kind for this that varies a bit from lot to lot. SAAMI accepts ±90 fps, about 3 times what the military accepts. But that flexibility allows a manufacturer to use some powder he has on hand rather than having to order and wait for a special lot to be made, provided this meets his own internal standards.
It should also be noted that SAAMI pressure and velocity test barrels have minimum chambers and specific bore cross-sectional areas and specific bore lengths that your gun may not match exactly. This, on top of factory velocity tolerances, both can contribute to you having different velocities than a manufacturer claims for his ammunition.
Bottom line: can you duplicate factory ammo? Sometimes yes, sometimes no.
