as a martial arts example, Ben teaches BJJ, that's his thing but he also teaches other arts well his sidearm of choice is the 9x19mm; however... once he hits the ground, the optimal sidearm caliber becomes 9x17mm; and for reasons he sees as viable... he chooses the 9x19mm as a universal caliber ...despite that shift in dynamic, because the 9x19mm still works it's just the guns are a little bigger