Alpo, as Tranter said, excellent explanation of the internal balistics that occur when firing a round.
The Venturi effect discribes what happens with fluids, not gasses.
Say for instence your using an eductor for foam for flammable liquid fires.
The eductor has an inlet and an outlet for water to pass through. As the water passes over a small hole in the tube it "sucks out" the foam and mixes it with the proper amount of water. This is an example of The Venturi effect.
Hope that helps