I recently answered a question on the WorldBuilding forum about grenades and bullets. One of the things that came up was that I argued smokeless powder in a rifle round could detonate, but was challenged on that. Commenters said that smokeless powder only deflagrates during normal use.
This, however, leaves me with a question. How can we accelerate a bullet to supersonic speeds using only a sonic speed pressure wave? As the bullet approaches the speed of sound, shouldn't the pressure wave be pushing the bullet less effectively? It strikes me that a bullet traveling at the speed of sound should not be able to be pushed by a pressure wave at the speed of sound.
How does this work?