This is a common misconception. If the powder you are using is getting the best speeds with with a long barrel, that is the powder you will get the best speeds with from a shorter barrel. Using a faster powder for shorter barrels is a myth
I didn't mean it as in the shortened barrel length means everythings different and he'll have to find another load, rather my concern is that if the barrel is too short for a powder and that powder doesnt finish burning and hence muzzle flash is produced then that load doesnt do what he intends for it to do as the pressure levels never get quite high enough.
of course the load might still shoot well, but its a concern.
or am i still off on that train of thought?