That is an interesting question.Interesting question! From what I can find, the gasses leaving the barrel but not interacting with the atmosphere would be around 2.25 to 3.0 liters for a 50gr charge. Of course the weight of these gasses is the same as the weight of the powder. Also, the volume of these gasses is figured at room temperature.
This is black powder so it's generating about 1/3 the gas volume of smokeless powder. Thanks for this info!