I think we are saying the same thing in different words re expanding gas and accelerating bullets down barrel. We also agree that .357 would yield more power than .38 in same length barrel, even a snubby. The only thing I'm not too sure about is exactly when the powder is fully consumed. Some loads with very slow burning powder will eject unburned powder granules from the muzzle, clearly showing that not all powder is consumed instantly in the case before bullet begins movement.
Good shooting and be safe.