A week at Barrett's long-range training courses will blow away the misconceptions surrounding the company's .50-caliber rifles while teaching you the fundamentals of big-bore, extreme-distance shooting.
When dealing with two similar calibers like the 9x21 mm and the 9 mm one must be particularly cognizant that the ammo used matches the caliber stamped on the barrel to lessen the possibility of potential damage to yourself and/or your firearm.