You're right. I would not depend on two different powders delivering the same mix of propellant gasses. Single base and double base would almost surely yield different gas mixtures.
Unless there is some great simplifying assumption that I don't know about, working out the pressures and temperatures from basic principles will produce a major headache.
Comparing a mix of gasses at one pressure, temperature and volume with the same mix at a different temperature, pressure, and volume is no problem.
Edited to add: Maybe it's not as horrendous as I thought. You should be able to get a temperature from the emission spectrum of gas as it exits the muzzle. You know the pressure at that point, and should be able to work back to the peak pressure and temperature. Maybe. I guess.
Last edited by denton; 06/11/18.
