MtGun44,
My understanding George Luger is credited with inventing the stepped chamber for 9 mm. Instead of having a tapered chamber, he used two straight sections in his chamber. The approximately one third of the chamber next to the riflings was tight enough to align the bullet and was the area that produced the gas seal when firing. The rest of the chamber was a straight cut 0.1 mm larger diameter than the front of the chamber. So there is s 0.1 mm step in the chamber about 2/3's the way into it.
The stepped chamber definitely works well. My knock on it is the step it puts in brass I would want to be reloading. Now I'll say in fainess I haven't owned a stepped chamber so I really don't know if the resulting stepped brass is a problem. I just didn't want to buy a $700 gun and find out. So I passed.
I imagine some folks might not notice the stepped chamber until they bought it, hence the "heads up".