If you have 1 gram of a propellant, a single base (nitrocellulose) shotgun powder, that is known to produce a max pressure of 9000 psi when pushing 30 grams of payload out the end of a 75 cm long barrel or tube at 360 meters/sec, how do you calculate the explosive velocity (burn rate) of the powder? The volume is constantly changing because the payload has started to move out the barrel before the pressure reaches max value. Assume an initial volume for the propellant of 1 cc. and the payload to have a volume of 3.5 cc. And if you used twice as much powder (2 grams) would the exit speed increase by a factor of 2? And using 2 grams of powder, would the max pressure double or exponentially increase? And would the explosive velocity be the same? How do you do these calculations? Or can you recommend the BEST book for this kind of internal ballistic problems?