Some powders burn "dirtier" than others, so you'll have more fouling. Shooting cast bullets will also cause more fouling from the bullet lube, etc. Factory ammunition is loaded with what is referred to as "bulk" powders, in which they use ballistics labs to work up the load for each lot, using the velocity and pressure parameters for each loading. Reloaders use what are referred to as "canister" powders, where the powder is blended to give the same general burn rate for the given powder, within the parameters set by the manufacturer/distributor for each powder.
There is variation within the parameters, so the same powder could differ from lot to lot. If one lot is blended to the specifications at the top of the parameters, and the next lot is blended to the bottom of those same parameters, then there will be significant variation between the two lots and resulting ballistics changes in both pressures and velocities. That's why the loading manuals tell you to check your loads when you change lots of powder, and why I try to buy my powders in 8 pound jugs, or full cases of 32 pounds from the same lot. It saves a lot of reworking loads.
There are other variables involved, too. You'll notice that once fired brass is cleaner inside than twice, thrice, etc. fired brass. When the round is fired the second, third, etc. time, it takes some of that residue with it, and if you don't clean your primer pockets, it also takes some of the residue from there as well. It's a very small amount each time, but it accumulates as additional rounds are fired.
Some of the newer powders are advertised as being cleaner burning, and they seem to be. Unique has a reputation for leaving residue behind, but the newer Unique doesn't seem to be as prone to this as the older batches were. You can experiment with other powders to see if they're cleaner, but only if having to clean your firearm less often is that important. To me, performance is more important. A perfect example is I use a lot of Bullseye for my cowboy action rounds, but Clays burns cleaner and leaves less residue behind. I like Bullseye and have been using it for about 40 years or so. It gives me confidence, so I use it and understand that I'll have to clean my guns more often.
Hope this helps.
Fred
