At the risk of putting words in Bowser's mouth, what I think he means is that a certain weight of powder will yield a certain number of molecules of gas, which corresponds to a certain volume of gas at STP.

For example, I looked up the amount of gunpowder in a 9mm round. it is about a 1/3 of a gram. If we make the assumption that this is all carbon (that the oxidizer mass is negligible) we will wind up with 1/36 of a mole of CO2 which at STP is 22.4/36 = 0.62 liters of gas.

Of course, this pint of gas at freezing temperatures and 1 atmosphere is crammed into a tiny volume at very high pressure and temperature. A great deal of energy is available to move the bullet.