I've been watching a lot of my WWII footages yesterday ...
That ground effect is particularly visible with the Stug III , due to the low profile and the short distance between the muzzle and the ground ....
The mini fan I used in the "smoking gun" thread is lacking the pressure and has too much inertia to recreate that burst of energy ....
So I agree with you that CO2 is the way to go ....
I have a brand new Heng Long Stug III in the box , which I was planning to modify ...
That will be the perfect test bed for such a system ...
In any case ; It should be a fun project ....