Which of the following is a common caliber used in semiautomatic pistols?

The most common caliber used in semiautomatic pistols is 9mm. This caliber is widely favored for its balance of manageable recoil, sufficient stopping power, and high capacity in magazine designs. It is used by law enforcement agencies, military organizations, and civilian shooters alike, making it one of the most popular handgun calibers worldwide.

While .40 caliber is also a common choice for semiautomatic pistols and offers different characteristics in terms of recoil and power, it does not reach the level of popularity that 9mm does. It's important to note that both .40 caliber and 9mm are prevalent in firearms, but the higher use and recognition of 9mm make it stand out even more.

In the context of the question, mentioning both .40 caliber and 9mm together highlights the presence of multiple commonly used calibers in semiautomatic pistols. However, if solely focusing on the most widely recognized and utilized caliber, .40 is secondary to the prominence of 9mm. Therefore, the inclusion of both in the context of common calibers does not diminish the status of 9mm as the standout choice.