[muddober]

Both Jim K and Helix are pretty much right on but I will simplify it a bit. First and foremost D2 cannot be and obviously is not zero, that part you have not figured out, but it is the basis of your question, yet you know and understand I am right. So where to look for delay (you haven't in your equation) is the length of time it takes to compress the gas to a point that the pressure becomes great enough to operate the action, which by then the bullet is long gone. Carefully read both Jim k and Helix, paying special attention to Helix.

Ron