[Brian Pfleuger]

As with most things, there is no absolute.

In most instances, powder continues to burn all the way down the barrel and out the muzzle. There aren't many instances (they do exist) in rifles where the entire, 100% charge is burned inside the barrel before the bullet exits. QuickLoad can give you predictions of which ones will under what circumstances and there's plenty of examples where you DO get 100% burn but it's the exception, not the rule, and I know of NO instance under normal circumstances where the powder burns 100% inside the case. Trail Boss under jacketed bullets in a rifle cartridge comes close, but even then the bullet (and some powder) moves a few inches before 100% burn is complete.

On barrel length, it is GENERALLY true that the powder which produces the fastest speed in a long barrel will also produce the fastest speed in a shorter barrel. This holds almost universally true until the length difference gets extreme, not an inch or 2 or 3 but 10 or 12 or 15 or more.

You've basically got three "zones".

Long barrel, slow powder makes fastest speed. SUPER short barrel (I mean like 2 inches) and some fast powder makes fastest speed. As the long barrel gets shorter and the short barrel gets longer, the two speeds get closer and closer until they eventually meet and cross. Where that is, there are too many variables to pinpoint without specifics.