The powder smoke just doesn,t recoil when the momentums of the gun and the bullit are in balance. To what could it recoil ? The bullit ór the gun (+ box aso).
Recoil would mean an imbalance of momentums between the bullit and gun (+..) and when there is balance what gives the additional momentum to the gunpowder (other then gunpowder plus further content of the expanding space between bullit and gun with/after ignition?.
Be aware that the massbalance for the gun on itself may be at one side of the gunpowder but the gun+box+ disk and all can have the massbalance in front of the barrel opening.
The gunpowder also has a massbalance somewhere plus the system as a whole. So it starts with four G,B,P plus total (G+B+P)
If these are considered massmiddleplanes these planes can be seen as continued through and ouside the barrel and box.
From P (powder) the momentums Pg and Pb must balance. But also Ppowder must be zero.
A recoil wouldn,t keep it to the reference P(powder) but choose the side of the gun .. Then the bullit and powder must have same momentum as the gun......Where would the energy come from then. That would make it a collision instead of a shot.