Which of the following is NOT a common rifle or handgun caliber?

The choice of .25 as the option that is not a common rifle or handgun caliber highlights a distinction in the usage and popularity of various calibers in hunting and firearm contexts. The .25 caliber, while it does exist, is not as widely recognized or utilized compared to the other options presented.

The .44 caliber is a common choice among both handguns and rifles, often favored for its stopping power and versatility. The .30-06 is a well-known rifle cartridge, particularly popular for big game hunting due to its performance and historical significance. The 9mm is one of the most common handgun calibers in the world, widely used for personal defense, law enforcement, and competitive shooting.

In contrast, the .25 caliber, although it can be found in some smaller handguns, does not hold the same level of popularity or prevalence as a primary choice for hunting or self-defense compared to the other listed calibers. This difference in commonality among shooters makes .25 the correct answer in this context.