PS Just for a quick cocktail-napkin calculation--assume 1 gram of carbon propellant combines with oxygen to form CO2. I will ignore the other substances in the propellant and how much gas they produce.

C has a gram-molecular weight of 12, so 1 gram of C yields 1/12 of a mole of CO2. A molecular gas is mostly empty space, so the oxygen is irrelevant, it is only the number of molecules that matters, not how big they are. A mole of any gas occupies 22.4 liters at Standard Temperature and Pressure, so the gas produced is equivalent to 1.9 liters at STP.

So for just a very rough estimate, I would say a typical bullet charge produces a couple of quarts of gas at STP. Of course, in the barrel, that gas is extremely hot and highly compressed, so the pressure is enormous.