Are you looking at volume in terms of molecules of gas produced by molecules of powder?
Using a simple nitrocellulose molecule and assuming as complete combustion as possible.
C6H8(NO2)2O5 => 2CO2 4H2O 2N2 CO (remainder 4 Carbons).
That gives you 9 molecules of gas for every molecule of nitrocellulose. But you can't really compare the volume of a solid with the volume of a gas, as the volume of the gas is both temperature and pressure dependent.
Or are you asking for the equivalent volume of gas produced at normal atmospheric pressure?
Because then we get into pV=nRT and right now it is too late for me to even try to do that math with any sort of fidelity.
9 molecules of gas for every molecule of nitrocellulose means I (or someone with plenty of time) would have to compute for each type of molecule separately and add the partial pressures, adjusting volume of the final product to normal pressure to one ATM.
Let us know what you really want to know, I'm sure we've covered it before.
I pulled the following from wikipedia as it may explain detonation velocity a bit better, which might be what you are really asking for anyways.
Currently, propellants using nitrocellulose (detonation velocity 7,300 m/s) (typically an ether-alcohol colloid of nitrocellulose) as the sole explosive propellant ingredient are described as single-base powder.
Propellants mixtures containing nitrocellulose and nitroglycerin (detonation velocity 7,700 m/s) as explosive propellant ingredients are known as double-base powder.
During the 1930s triple-base propellant containing nitrocellulose, nitroglycerin, and a substantial quantity of nitroguanidine (detonation velocity 8,200 m/s) as explosive propellant ingredients was developed. These propellant mixtures have reduced flash and flame temperature without sacrificing chamber pressure compared to single and double base propellants, albeit at the cost of more smoke.
Machine guns are awesome until you have to carry one.