On a more prosaic airgun level, if you are, say, Crosman, and you want to use the same parts for a carbine (2250) and a pistol (2240), you pick the right-hand option. Crosman are a master class in using the same basic parts in a wide variety of guns, often for decades. Some of the Chinese makers are now at the same level, producing the same base design in PCP, multi-pump, CO2, and rifle and pistol variants.