There’s three variables in the thermodynamic formula referred to as simple physics. This formula is valid in certain and certainly in what we’re discussing.
Pressure temperature and volume.
If volume is constant then a rise in temperature will subsequently cause a rise in pressure.
If volume is not held constant say absolute temperature is raised by ten percent and volume of gas in a confined space is also increased by ten percent, what is the resultant pressure?
Similarly if a solid sublimes or due to combustion creates gas, depending on the type of the gas produced, different amounts, in volume, of gas will be produced.
An extreme example. A frozen block 6 grams of hydrogen and a frozen block 6 grams of CO2 are thawed. The the gas produced will be 6 grams. Now if those gases are contained in one cubic meter, and the temperature is zero degrees C. The pressure inside the container of hydrogen will be greater than the container with CO2.
Now, consider the different chemical mixtures of powder.
Yes, absolutely. Because there are more moles in 6 grams of H2 than in 6 grams of CO2.
But when we talk about the combustion of powder charges, the resultant gases are remarkably similar, even though the powders may be of dissimilar burn rates. All of our powders are composed nitro glycerin combined in some percentage (possibly zero) with nitro cellulose.
No, the gases produced by burning Bullseye are not identical to the gases produced by burning H870. But they are close enough to give us two or three significant digits of accuracy. With all the other vagaries involved firing a rifle cartridge, distance from the lands, chamber dimensions, lubricity of the bullet, rounds since last cleaning, actual internal volume of each piece of brass, actual temperature of the barrel at the time of the shot, etc, etc, etc. A couple of significant digits is as close as we can come.
The simple point being you can not get to 65,000 psi by burning powder without a spike in temperature, and it takes a higher temperature to make 65,000 psi than it does to make 55,000 psi.
