In 1980, the 9×21mm was standardized. It goes by many names. Regardless of which moniker is applied, this is merely the 9×19mm Luger with case length increased by 2-mm (0.080-inch) but using almost the same cartridge overall length. As standardized, pressure is a bit less in the 9×21, compared to the 9×19. USPSA and IPSC shooters once used this chambering as a means of assuring that their high-pressure handloaded munition did not get used in any standard 9×19 chambered pistol. This was a good idea because many of them used loads combining very light bullets and very high velocity to reduce recoil while still achieving the critical Major-Power-Factor criteria. Such loads required extremely high pressure. This standard was based on momentum, so the lighter the bullet the lower the felt recoil and the faster the shot-to-shot recovery time for any given power factor. When loaded to SAAMI-specification pressure ballistics of the 9×21 and 9×19 are indistinguishable. As with any pistol round headspacing on the case mouth, best practice is to apply a taper-crimp after seating the bullet. This helps to lock the bullet in place against chambering forces that might otherwise drive it into the case. I cannot too-strongly recommend getting a second seating-and-crimping die so you can have one adjusted to only seat the bullet and the second adjusted to only taper-crimp the case mouth. Generally, attempting to do both operations in one step is a recipe for damaged and destroyed cases.