9mm brass made by forced extrusion rather than typical pressure drawing causes there to be that visible "ledge" about halfway down the case wall. It's just cheaper to make them using a forced extrusion method. You can experience the separation that looks like a cleanly cut case. This is less common in typical breech lock service pistols and more common in blow back designs such as the 9mm AR uppers. Freedom Munitions, Maxxtech, IMT, Ammoland - some of the ones that make these. Just one more thing to look for to cull out of your brass for reloads. It's not for bullet setback. The step is just an artifact from the manufacturing process. If you are using this brass in a PCC you run the risk of case separation, don't use it. It is cheap brass and prone to failure. It is my understanding that at least some of the manufacturers have stopped using this type of brass due to complaints of case failure in non-breech locking blowback weapons.
