Yes. For example, on some trombone leadpipes (I’m using trombone leadpipes as examples because they can be pulled and visually inspected easier than most other brass instruments) the end of the mouthpiece basically sits at the beginning of the expansion and provides no pressure node of any s ...

Gap can be thought of as the distance to a venturi. A venturi (constriction in a tube system) creates a pressure node. In trombone leadpipes, there’s really no sharp edge creating a definitive spot, but there’s still a venturi. In other brass, sometimes there is a sharp, definitive spot, sometimes th ...