Part of the issue with a design like this would be that storing compressed gases is very bulky and prone to catastrophic total failure (very bad for a weapon being abused by grunts). Gunpowder in cartridges already contains an propellant and oxidizer, and per mass you really aren't going to gain anything from going with a gas based system.

Heat management would also be a huge issue, spent casings carry away a surprisingly large amount of heat, and even then most guns are unable to sustain continuous fire rates for long. This was a serious issue with case-less ammunition designs for instance.