Completely a non-issue. By focusing on burning all the powder, you're sacrificing a large amount of potential gas volume and will see the pressure spike and quickly drop. In contrast, what you want to happen is the pressure rise to the max allowable (this will happen in the first inch or two of bullet travel) and then stay as high as possible for as long as possible. The short barrel simply means that you'll have a high pressure when the bullet exits the barrel. You're giving up a lot of velocity compared to a longer barrel already, the last thing that you want to do is make things even worse by sacrificing the little pressure that you can sustain.