The muzzle blast from a short barrel has nothing to do with powder still burning, and by going with a faster powder in a shorter barrel, you're giving up velocity. The pressure curve for the bullet/cartridge combo is determined within the first 3 inches of barrel. After that all you have is the residual pressure curve. The shallower the decline of the residual pressure after the pressure peak, the higher the resultant velocity, regardless of barrel length.
What you're looking for in a powder/bullet/cartridge combination to achieve best velocity is a burn rate that reaches peak pressure with the highest charge weight possible. That will produce the most gas volume and sustain the highest pressure for the longest duration. If the powder is too slow, you run out of case capacity before you reach peak pressure. If the powder is too fast, you hit peak pressure with too small a charge to generate sufficient gas to maintain pressure. Declining pressure means decreasing bullet acceleration. With an optimum combination, there will still be fairly high pressure behind the bullet when it leaves the barrel in a short barrel and that pressure is what creates the muzzle blast, but it will give you higher velocity than a combination where the pressure has already dropped significantly.
