Roller locking is a system that is not used in many guns and often confused with roller-delayed blowback - which is understandable, given the similarities between the systems. Roller locking is really a sub-type of flapper locking mechanism. It was most significantly used in the MG42, and also in the Czech vz.52 pistol. In essence, it uses rollers in place of flaps to lock the bolt and barrel securely together during firing, and depends on an external system (short recoil, in the case of the MG42 and vz.52) to unlock before it can cycle.