Let's look at this step by step:
1) First, all the powder that's going to burn will burn within inches of the case mouth, much less than a foot. Fast-burning handgun powders will burn within less than an inch; the slowest burning rifle powders within 2-4 inches.
2) If the pressure is correct for the powder in question then almost all will burn. Let't call it 99%.
Some rifle powders are designed to burn best at 50,000 psi or so, others (usually the slower ones) at 60,000 psi. This is assuming we're talking about conventional bottle-necked cases designed for modern bolt-action rifles.
My own experiments, as well as those of many other people, indicate that the "best" (producing the highest muzzle velocity) powders for a given cartridge and bullet weight will be the best powders in any legal barrel length, which in the U.S. is 16".
The powder/bullet combination that will lose the least velocity in a short barrel is a slow powder with a heavy bullet.
